add a rust API for defining struct types and ffi functions from safe rust without loading a dynamic library

[Skgland]

primarily adds functions Machine::register_struct and Machine::register_function

Machine::register_struct

This is the rust counterpart to to ffi:foreign_struct/2 in prolog.
It allows registering a struct that implements the unsafe CustomFfiStruct trait.

Machine::register_function

There is no prolog equivalent.
Related to use_foreign_module/2, tough without loading a dynamic/shared library and one function per call.

https://github.com/Skgland/scryer-prolog/blob/c1a79f3598c8bd56f7d5a8b3769548ee1bd17cea/tests/scryer/ffi.rs#L373-L388

Todo

• C compatible API
• WASM compatible API

Help wanted

• auto-load library(ffi) (or make loading it unnecessary), see https://github.com/Skgland/scryer-prolog/blob/c1a79f3598c8bd56f7d5a8b3769548ee1bd17cea/tests/scryer/ffi.rs#L373
• When registering a function from rust the corresponding ffi:'name'/n predicate as with ffi:assert_predicate() should be created, see
  • https://github.com/Skgland/scryer-prolog/blob/c1a79f3598c8bd56f7d5a8b3769548ee1bd17cea/src/ffi.rs#L1265
  • https://github.com/Skgland/scryer-prolog/blob/c1a79f3598c8bd56f7d5a8b3769548ee1bd17cea/tests-pl/ffi_static.pl#L2

Future Work

• a derive macro to automatically implement CustomFfiStruct
• a predicate in the prolog ffi moduld for ad-hoc calling of functions via function pointers
  something like ffi:call(FnPtr, ArgTypes, RetType, Args, Ret) (suggestion for a better name appreciated) after this PR most things needed should be in place already
• it might be nice to split use_foreign_module to have seperate predicates for
  • loading a library (get a library handle)
  • looking up symbols in the library (using the library handle)
    • get a function pointer (without defining it as a prolog predicate)
      • can be used with ffi:call/5 above
    • get a pointer to e.g. a static
  • defining prolog predicates for ffi funtion pointer
    • allow defining the predicate in a different module besides ffi
    • allows the predicate to have a name different from the function symbol
      e.g. allow loading plugins that all define the same entry-point/callback functions