Deprecate the fixed-width integer aliases

[gnzlbg]

We do guarantee that the fixed-width integer types {i,u}{8,16,32,64} are layout compatible with C's {u,}int{8,16,32,64}_t.

There is no need to provide type aliases (e.g. libc::uint64_t) for these, in the same way that we don't provide a libc::c_bool type alias for bool.

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Note that c_float and c_double aren't necessarily layout compatible with f32 and f64.