Recursive-descent calculator parses '(1 + 2) * 3 + 4' = 13. Two parser
bugs fixed:
1. parse-let now handles inline 'let rec a () = ... and b () = ... in
body' via new (:let-rec-mut BINDINGS BODY) and (:let-mut BINDINGS
BODY) AST shapes; eval handles both.
2. has-matching-in? lookahead no longer stops at 'and' — 'and' is
internal to let-rec, not a decl boundary. Without this fix, the
inner 'let rec a () = ... and b () = ...' inside a let-decl rhs
would have been treated as the start of a new top-level decl.
Baseline exercises mutually-recursive functions, while-loops, ref-cell
imperative parsing, and ADT-based AST construction.
giles
ffa74399fd