bootstrap:types: fix bin acc resolution when field not found
lib/n/builtins:String: Index_byte and Index
bootstrap:cprinter: support assignment '_ = expr', without let
bootstrap:unify: clean-up and fix unification with literals
bootstrap:cprinter: call through dyntable directly

Do not generate "defintf members" stubs. This is simpler, faster (a
bit), generates less C code.

Also, this change was necessary to properly call methods defined on a
"parent" of the intf of the dyn itself. The method prototypes are not
copied into the descendant intf, but referenced directly from the
parent. In the codegen, we were blindly getting the prototype from the
parent. But we were also reusing the "defintf member" stub generated *for*
the parent intf, but they have different dyntables! We had an entirely
bogus cast in the dyn call code before.
bootstrap: support passing down vararg to regular vararg functions

Avoids the whole printf/vprintf business that we had inherited. It is in
fact unnecessary.
lib: improvements path and builtins strings