Make imported functions inexact

[kripken]

Defined functions remain exact, but imported ones are inexact.

This is a step along the recent Custom Descriptors spec changes.

• New RefFunc::finalize and Literal::makeFunc variants get the module, and look up
  the type there.
• New Builder::makeRefFunc variant gets a Type and applies it. The HeapType
  variant does a lookup on the module (so the Type one is more efficient/applicable
  if the IR is not fully built yet).
• ReFinalize now updates RefFunc types (following the pattern of a few other places).
• C and JS APIs now assume RefFuncs are created after imported functions (so we can
  look up the type of the import; see changelog, this seems the least-annoying way to
  update here, avoiding new APIs, and less breakage for users - hopefully none, all our
  tests here pass as is).
• wasm-split adds a cast when a function becomes an inexact import.
• Fix GUFA to handle inexact function literals.
• Update types in passes and fuzzer as needed.