Provide support for `fast` integer types

[cramertj]

libc currently provides a number of convenient integer type aliases. However, the least and fast integer types (e.g. int_fast16_t, etc.) aren't currently included. Is there a reason for their exclusion, or is this just awaiting an implementation?

[alexcrichton]

Nah just awaiting an implementation!

[kellda]

After some research in apple's libc, musl and glibc, I found that [u]int_least(n)_t == [u]int(n)_t. For [u]int_fast(n)_t, however, it's more complicated:

bits	8	16	32	64
apple	8	16	32	64
glibc 32	char	int	int	long long
glibc 64	char	long	long	long
musl	8	32	32	64

How to know which typedef to put in this crate ?

[kellda]

@alexcrichton or someone else could you mentor me to implement fast types alises ?

[workingjubilee]

In #1838 it was noticed that the "least" aliases resolve exactly to our types, so they were omitted for the same reason that the fixed-width aliases were deprecated.