Chapter renumber, and split ch 17

Renumber chapters 10a, 10b, 11, 12, 13, 14 -> 10, 11, 13, 12, [11,14].
Split ch 17 into a 17a and 17b; shuffle around im/mutable syntax to be consistent, and present it more slowly.
THe new immutable syntax is pushed through all later chapters

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+# Chapter 17b grammar additions
+
+Start with [Chapter 17a's declaration syntax](../../chapter17a/docs/17-grammar.md).
+This chapter adds inference, assignment shortcuts, conditional expressions, and
+`for` loops. Access and binding permissions keep the same meaning.
+
+```text
+inferredDeclaration = ("var" | "val") ("!" | "~")? identifier "=" assignment
+assignment          = expression (assignmentOperator assignment)?
+assignmentOperator  = "=" | "+=" | "-=" | "*=" | "/=" | "&=" | "|=" | "^="
+                    | "<<=" | ">>=" | ">>>="
+expression          = bitwise ("?" assignment (":" assignment)?)?
+prefixUpdate        = ("++" | "--") identifier
+postfixUpdate       = lvalue ("++" | "--")
+forStatement        = "for" "(" declarationOrExpression? ";" assignment? ";"
+                      assignment? ")" statement
+```
+
+Declarations can contain several comma-separated names. `var` permits
+reassignment and `val` fixes the binding; both retain the initializer's reference
+permissions. Postfix updates also support fields and array elements. Prefix
+updates currently support local names. See the README for evaluation order.