579b76b7cb Introduce ToU64 conversion trait (Tobin C. Harding)
Pull request description:
The idea for this was pulled out of Steven's work in #2133
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)`.
ACKs for top commit:
Kixunil:
ACK 579b76b7cb
apoelstra:
ACK 579b76b7cb successfully ran local tests
Tree-SHA512: 2eaddfff995987a346e052386c6dfef3510e4732e674e3a2cfab60ee391b4cce1bf7ba4fb2dfd4926f8203d7251eea2198ccb61f0b40332e624c88fda4fa7f48