Should the type of function_pointer in GuestFunctionDefinition be changed from i64

[simongdavies]

The field function_pointer in struct GuestFunctionDefinition is defined as a i64.

This requires that a function pointer passed to register_function is cast to i64

When running clippy on a guest that casts from a func directly to i64 (e.g. myfunc as i64) results in the clippy error "casting function pointer myfunc to i64" and the suggested fix "try: myfunc as usize" which does not work.

The options are to disable the clippy check ([allow(clippy::fn_to_numeric_cast)]) or to double cast the pointer (e.g. myfunc as *const() as i64 or myfunc as usize as i64).

Should the definition of the field function pointer be changed to either usize or fn(&FunctionCall) -> Result<Vec<u8>>

[syntactically]

Hopefully we'll get rid of GuestFunctionDefinition entirely soon :)