reftypes: func.ref, call_ref, return_call_ref

* module/wasm/wat.scm (parse-wat): Add support for ref.func, call_ref
and return_call_ref instructions.
(parse-wat) <parse-heap-type>: New procedure.
(parse-wat) <parse-ref-type, parse-val-type>: Use it. Allow both
nullable and non-nullable reftypes.
(parse-wat) <parse-params>: Allow reftypes as parameter types.
(parse-wat) <parse-type>: Fix call to parse-func-sig.
* module/wasm/resolve.scm (resolve-wasm): Use resolve-type to obtain a
typeidx for call_ref and return_call_ref operands.
* examples/hof.gwat: Add example program from function-references
proposal.