More documentation to follow, but in brief:
- IO is now a synonym of IO' FFI_C
- IO' is parameterised over an ffi description, which explains which
types are allowed in foreign calls, and what a foreign call target is
(in C, that's a String giving the function name, for example)
- New function "foreign" for building a foreign call, given an ffi
description, call target, and the type of the call. The type is enough
to build a structure which the compiler uses to generate the call.
The type of main now needs to be IO' x (), where x can be any FFI
description.
There is currently only an ffi description for C; Javascript will
follow. FFI_C should also work for the llvm backend (it's really about
the calling convention, not the backend specifically). Small changes
will be needed to the various code generators.
