Chapter4 (#8)

* Initial work on chapter 4.
Updated language grammar for chapter 4
Enhanced Parser to implement comparison operators
Parser now allows setting 'arg' as input parameter to program
The StartNode now has a tuple as result with two values.
We set up two projection nodes as output of Start - the first is bound to scope as '$ctrl' and second as 'arg'.
Return node now obtains the ctrl input by looking up '$ctrl' in scope; thus it gets the projection node.
Added TypeTuple - represents a tuple of values. This is the type of Start.
Added BoolNode - for comparison operators.
Added MultiNode - interface for nodes that generate tuple results. Start implements this interface.
Added ProjNode - projections to extract values from Start node. Each projection takes a MultiNode and an index value.
Initial writeup of chapter 4 in README.
GraphVisualizer now searches node outputs as well to locate all nodes

* Add TypeControl.
The first member of Start's tuple is of TypeControl.
Correct the projType() method

* Quote port names in edges as unquoted $ctrl is not valid.
Revise how we decide an edge is a control edge so that it works correctly for projections.

* We used to compute initial type in Node but this is problematic for new node types such as
ProjNode and BoolNode as the construction is not yet complete when Node calls compute().
So we have to move compute() call down to the sub type's constructor.
Add some tests for comparison ops.

* Describe the $ctrl name binding and how it represents the Ptri net model.
Add a diagram to show why we need algebraic manipulation of expressions.

* Implement a simplification for expressions such as
(con1 + arg) + con2 -> arg + con3
con1 + (arg + con2) -> arg + con3

* Add diagram to show what the graph looks like post peephole

* Explain projection nodes

* Cleanup Chapter 4, add peepholes.

Add like 6 peepholes.  Replace the 2 prior ones with 1-liners.
BoolNode: replace the switch with 6 1-line classes.
Giant comment discussion on idealize() calling convention.
TypeInteger shorten isTop and isBot, and use them instead of calling "equals()".

* 2 more peeps, no tests yet

* Cleanup Chapter 4

`peephole()` kills nodes killed by `idealize()` calls... and works recursively.  This requires keeping "mid-peephole" nodes alive.  The edge-swap code in Add and Mul now keeps their inputs alive mid-swap.

MultiNode is a class, not an interface.

Move the initial 'compute()' to peephole, instead of requiring each constructor to do it.  Start and Constant remember their original types, even if the `_type` field changes.

`kill()` flags nodes as DEAD.  Print DEAD nodes with "DEAD".  Change recursive `_print()` to a tik-tok style, so each individual print does not need to handle DEAD printing.  This printing will soon need a "visit" bit to avoid infinite recursion.

Projs pick up a debugging name.

GraphViz: combine Multis and their Projs.  Drop Constant inputs from Start.  Fix coloring Control edges red.

* minor WS cleanup

* Cleaned up Node.peephole

Use a smart DCE call after peephole.  More comments.  Tidy interfaces.

* minor comment cleanup

* But the proj nodes inside a table

* No need to set column span on multi node as we put the proj nodes in a table

* Update the dot diagrams

* Rename method to make it more obvious to the reader

* Start to add a description of peephole

* Rename del() to delUse() and make it an instance method as it reads better that way.
Add new method addUse() again to improve readability.
Adjust removeDeadCode() to make the intention clear.

* Update docs explaining how peephole works
Update the diagrams

* Fix errors

* Add diagram to show what the graph looks like post peephole

* 2 more peeps, no tests yet

* Cleanup Chapter 4

`peephole()` kills nodes killed by `idealize()` calls... and works recursively.  This requires keeping "mid-peephole" nodes alive.  The edge-swap code in Add and Mul now keeps their inputs alive mid-swap.

MultiNode is a class, not an interface.

Move the initial 'compute()' to peephole, instead of requiring each constructor to do it.  Start and Constant remember their original types, even if the `_type` field changes.

`kill()` flags nodes as DEAD.  Print DEAD nodes with "DEAD".  Change recursive `_print()` to a tik-tok style, so each individual print does not need to handle DEAD printing.  This printing will soon need a "visit" bit to avoid infinite recursion.

Projs pick up a debugging name.

GraphViz: combine Multis and their Projs.  Drop Constant inputs from Start.  Fix coloring Control edges red.

* minor WS cleanup

* Cleaned up Node.peephole

Use a smart DCE call after peephole.  More comments.  Tidy interfaces.

* minor comment cleanup

* But the proj nodes inside a table

* No need to set column span on multi node as we put the proj nodes in a table

* Update the dot diagrams

* Rename method to make it more obvious to the reader

* Start to add a description of peephole

* Rename del() to delUse() and make it an instance method as it reads better that way.
Add new method addUse() again to improve readability.
Adjust removeDeadCode() to make the intention clear.

* Update docs explaining how peephole works
Update the diagrams

* Minor cleanups

Split isUnused from isDead, with comments.
Rename removeDeadCode back to deadCodeElim after discussion; matches the historical compiler wording.
Fixup large javadoc example with a table.

* match doc and code

* Fix name of method

* Fix name of method

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Co-authored-by: dibyendumajumdar <mobile@majumdar.org.uk>

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