Introduce try_from_fn

[Fuuzetsu]

This is just from_fn that takes a function that returns Result<T, E> instead of T.

This means that when we have

StaticMap::from_fn(|v| foo(v))

and now foo becomes a Result, we could do

StaticMap::try_from_fn(|v| foo(v))?

For now the most ergonomic thing is

static_map! { v => foo(v)? }

which works but has all the usual annoyances of using a macro such as formatting problems.

One can't even go via array and try_map from stdlib and rewrap as it's still experimental.

Original ticket in enum-map: https://codeberg.org/xfix/enum-map/issues/148

[Fuuzetsu]

I guess it'd use rust-lang/rust#84277 once that's stable.