Implement `:func` ABI for calling GPUCompiler emitted code

[topolarity]

This ABI is always ccallable, unlike the :specfunc ABI which frequently is not.

However, this ABI comes with a trade-off of introducing allocations (almost) always for function entry/exit. For that reason, we should re-enable the :specfunc ABI as soon as we can on as many types as possible.

Resolves #70

[gbaraldi]

LGTM

[topolarity]

Superseded by #82 and #85