We already explicitly do not support 16 bit machines.
Also, because Rust supports `u182`s one cannot infallibly convert from a
`usize` to a `u64`. This is unergonomic and results in a ton of casts.
We can instead limit our code to running only on machines where `usize`
is less that or equal to 64 bits then the infallible conversion is
possible.
Since 128 bit machines are not a thing yet this does not in reality
introduce any limitations on the library.
Add a "private" trait to the `internals` crate to do infallible
conversion to a `u64` from `usize`.
Implement it for all unsigned integers smaller than `u64` as well so
we have the option to use the trait instead of `u32::from(foo)`.
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