Support conversion of primitive integer types in const context

[U007D]

Support primitive integer conversion via new const_conversion feature or when building with a version of the compiler where const_trait_impl has stabilized.

[ctrlcctrlv]

(Merged into MFEK@8271720 due to #189 (comment). You likely don't care and the maintainer does plan to return, just explaining the merge message above.)

[ctrlcctrlv]

Oh, here's something you might indeed care about though @U007D—you probably want to add MFEK@54b6a41 to your branch because otherwise your PR adds a warning.

[petoknm]

It would be nice to have const FromPrimitive as well, but I can't get it to work.

I tried to just add const to the FromPrimitive impl in the macro expansion. Any idea why it goes wrong?
Here's the error for u32.

Compile error

error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
   --> num-traits.rlib-c0584f9c87376f82/8271720/src/cast.rs:625:17
    |
563 | / macro_rules! impl_from_primitive {
564 | |     ($T:ty, $to_ty:ident) => {
565 | |         #[allow(deprecated)]
566 | |         impl const FromPrimitive for $T {
...   |
625 | |                 n.$to_ty()
    | |                 ^^^^^^^^^^
...   |
628 | |     };
629 | | }
    | |_- in this expansion of `impl_from_primitive!`
...
641 |   impl_from_primitive!(u32, to_u32);
    |   --------------------------------- in this macro invocation

As far as I can see, it should work, because the <u32 as ToPrimitive>::to_u32() is a const fn.
Feature const_conversion is enabled.

[cuviper]

Hi -- I'm not inclined to merge any unstable features in the API, especially because I don't want the maintenance burden if that syntax changes at all. So I'm going to close this for now, but you're of course welcome to keep experimenting in a fork, and please feel free to reopen or file a new PR when the language has stabilized this.