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.

Makefile

@@ -75,8 +75,8 @@ reader.o: reader.module.scm reader.scm utils.o
 sex-modules.o: sex-modules.module.scm sex-modules.scm reader.o utils.o
 	$(CHICKEN_C) $(CSC_FLAGS) $(MODULE_FLAGS) sex-modules.module.scm -o sex-modules.o -unit sex-modules -link reader,utils
 
-semen.o: semen.module.scm semen.scm sex-macros.o sex-modules.o types.o utils.o
-	$(CHICKEN_C) $(CSC_FLAGS) $(MODULE_FLAGS) semen.module.scm -o semen.o -unit semen -link sex-macros,sex-modules,types,utils
+semen.o: semen.module.scm semen.scm infer.o sex-macros.o sex-modules.o types.o utils.o
+	$(CHICKEN_C) $(CSC_FLAGS) $(MODULE_FLAGS) semen.module.scm -o semen.o -unit semen -link sex-macros,sex-modules,infer,types,utils
 
 sex-fmt-c.o: sex-fmt-c.scm
 	$(CHICKEN_C) $(CSC_FLAGS) $(MODULE_FLAGS) sex-fmt-c.scm -o sex-fmt-c.o -unit sex-fmt-c
@@ -97,7 +97,8 @@ sextest:
 	cp ./tools/sextest/sextest .
 
 SEX_TEST_PROGRAMS = hello-world lists comments unicode serialize features \
-                    feature-flags lambdas compound-literals closures fixpoint
+                    feature-flags lambdas compound-literals closures fixpoint \
+                    wildcards inference
 
 # Multi-module linking is checked end to end; see tests/modules/Makefile.
 check-modules: sexc