This flattens `FunctionInstance.FunctionType` into `Type` and `TypeID`
fields, where the former is known prior to instantiation. This helps
pave a way for integration between Wasm declared and host defined
modules.
This also clarifies that `FunctionInstance.String` was used as a lookup
key, by renaming and caching its impl. While at it, I renamed "null" to
"v" in its output as I had been using v for void noticing others were
doing that also. Moreover, null is easy to misunderstand as a bug.
Signed-off-by: Adrian Cole <adrian@tetrate.io>
757afc2c7a