Add a "platform-specific C types" module to std lib.

[graydon]

The C int type is not always equal to intptr_t, and there is always the joyful nature of long, long long, signed/unsigned char, and similar hilarity. Add a module with all the normal numeric typedefs in the C standard library and make it per-platform, so library bindings can discuss this stuff in its natural terminology.

[graydon]

Shifted to rustc. Possibly inquire with LLVM to extract these values (else require they come from the platform-specific os module)

[brson]

There's a std::ctypes module now, mostly empty

[graydon]

is this mature enough to close the bug and just expand the module as needed by libraries?

[brson]

I think so.