Add #[repr(C)] to all the things!
manual: fix link
Add detailed note about Substs to ty_enum
liblibc: don't use int/uint for intptr_t/uintptr_t

int/uint aren't considered FFI safe, replace them with the actual type they
represent (i64/u64 or i32/u32). This is a breaking change, but at most a cast
to `uint` or `int` needs to be added.

[breaking-change]
libgreen: use FFI-safe types
Stage #[repr(packed)] in std::rt