Refactor int_like! macro

The function names from and into are almost never used outside of From and Into implementations. This MR implements those traits, along with renaming those const fns to new and get.

Additionally, the atomic types no longer provide default as const fn, instead the trait Default. That means replacing AtomicSchemeId with the cleaner Once<SchemeId>, and in some places, the MR also removes unnecessary Once wrappers.

The last two commits simplify the way the scheme ID is passed to UserScheme, allowing AtomicSchemeId to be removed (which might change if some future code uses atomic scheme IDs).