implement type inference

Two things out of one mechanism. `_' as a type means "work it out from
the initializer", so (var n _ (strlen s)) stops needing size-t spelled
out; `type-of' hands a macro the type of an expression, so a macro can
dispatch on what it was handed rather than on what was declared. Both
read the same answers from two sides.

Algorithm W's core, intra-procedural, with the extensions C forces:

  - an unknown type, since (include stdio.h) brings in names we never
    parsed. Unification is consistency rather than equality, so
    anything touching an unparsed declaration stops constraining
    instead of rejecting a program that compiled yesterday;
  - the usual arithmetic conversions, since `+' is not a function of
    one type;
  - checking mode for initializers, since #(0 0) has no type of its own
    and takes one from its context. #(T : ...) is the way out of that.

What it wanted on the way:

  - what type a *name* has, which neither the typedef nor the tag
    database recorded. One table serves functions and variables, since
    a function type already has a surface spelling;
  - a scope chain, so a (var c int 9) inside a do ends with the block;
  - form-type, keyed by cons cell, so one form has one type;
  - macros expanded during the walk rather than before it, so type-of
    is answered in the scope the macro was written in.

Closures take the same machinery: a receiver whose type comes from a
call, captures written (name expr) and typed from the expression, and
conversion from a bare function wherever a closure is expected.

type-match grew `_' on the pattern side, since (closure ((int)) int)
and (closure ((float)) int) were separate clauses for one case.

utils.scm

@@ -101,6 +101,19 @@
 ;;; The file `parse-all' is currently reading. Bound by the reader
 (define current-source-file (make-parameter "<unknown>"))
 
+;;; What type a form has, once something has worked it out. Keyed by
+;;; cons cell like the sources above, so one form has one type: a body
+;;; typed at two instantiations has to be copied before the second.
+(define +form-types+ (make-hash-table eq?))
+
+(define (set-form-type! form type)
+  (when (pair? form)
+    (hash-table-set! +form-types+ form type))
+  type)
+
+(define (form-type form)
+  (hash-table-ref/default +form-types+ form #f))
+
 (define (set-form-source! form file line)
   (hash-table-set! +form-sources+ form (cons file line)))