ImportFunction: use FunctionType internally

[evacchi]

ImportFunction currently takes a list of param types and a list of return types.
However, there exist a FunctionType object that represents exactly these lists; it would be natural for ImportFunction to reuse this object internally; then expose a .functionType() member

It might probably still make sense for HostFunction (at least) to expose an overloaded constructor:

new HostFunction(module, name, paramTypes, returnTypes, func)

to reduce typing for explicit construction; otherwise it will always look like:

new HostFunction(module, name, new FunctionType(paramTypes, returnTypes), func)

[electrum]

Related, why do we have both HostFunction and ImportFunction? It seems that we could replace all usages of HostFunction with ImportFunction and remove it.

[evacchi]

the initial idea was that the user-facing API could be unrelated to the internal API, and possibly that the naming is clearer.

The original problem was that everything was called a Host* something even when something did not originated from the host; indeed ImportFunction is less confusing, but on the other hand is less intentional (our end users will always define host functions). It also pairs well with the @HostModule concept.

I am not strongly against the idea of dropping it, if we think it's clear enough. Maybe we can take a look at the naming in other runtimes.