Add integer truncation and extension methods

This provides `.truncate()`, `.saturating_truncate()`,
`.checked_truncate()`, and `.extend()`.

These only work within the same signedness (use `.cast_signed()` and
`.cast_unsigned()` to change sign).

The truncation methods only work to smaller (or equal) types.
`.extend()` only works to larger (or equal) types.

For the purposes of truncation and extending, `usize` is considered
larger than `u16` or `u8`. This is consistent with `From`/`Into`
conversions.

Adding these methods results in needing to update the output of one test
that gets a new method-name similarity result.

Co-authored-by: Amanieu d'Antras <[email protected]>