Implement TryFrom/TryInto

[milesand]

Currently conversion between BigUInt, BigInt and primitives use dedicated traits ToBigInt, ToBigUInt and {To|From}Primitive, and not Try{From|Into} traits in std. While the functionality exists, this prevents using big integer types in a generic context that requires the latter. So, it would be preferable to implement them.

Some concerns:

• It might be better to integrate ToBigInt and the like directly with Try{From|Into}. But I think that will cause breakage downstream, especially with {To|From}Primitive. Still, if there is a way to do this with out breaking things, this seems preferable.

• Minimum supported rustc version. Since Try{From|Into} have been stable only since 1.34, I think naively implementing these would require bumping the rustc version number. Is there a way to avoid this? I'm not sure since I haven't dealt with minimum rustc versions before.

[cuviper]

I wouldn't want any breaking changes to the existing traits, but we can add TryFrom separately for new enough toolchains. TryInto comes implicitly with TryFrom. I would also be more conservative with this to match core/std, e.g. not implementing from floating point types at all. (In the existing traits we convert from floating point with truncation.)

Minimum supported rustc version. Since Try{From|Into} have been stable only since 1.34, I think naively implementing these would require bumping the rustc version number. Is there a way to avoid this?

We can add a test in build.rs like ac.emit_path_cfg("std::convert::TryFrom", "has_try_from"), and then guard the actual implementations with #[cfg(has_try_from)].

[milesand]

Thanks, I'll work on a PR.