Add TryFrom impls

[milesand]

Closes #120

Currently contains:

• TryFrom translations of existing conversion traits.
• A minimal Error type used for all TryFrom implementations.

Additional implemetations (e.g. TryFrom<BigInt> for BigUint which skips clone()) could be beneficial. Also, no new tests have been added since all new impls are basically thin adapters around existing, tested methods. It may be beneficial to add them regardless.

[cuviper]

FWIW, I rebased this to resolve the conflicts. It looks good!

Additional implementations (e.g. TryFrom<BigInt> for BigUint which skips clone()) could be beneficial.

I'm still thinking about this before merging, because it would be a minor breaking change to add it later. For example, given x: BigInt, x.try_into::<BigUint>() will auto-ref right now, but will consume x if we add the additional implementations later. This could easily break code that still needed x.

I'm somewhat inclined to add by-value TryFrom<BigInt> and TryFrom<BigUint> in all cases, even going into primitive integers, so they are useful in generic contexts like T: TryInto<u32> without forcing references.

[milesand]

I'm somewhat inclined to add by-value TryFrom<BigInt> and TryFrom<BigUint> in all cases

I pushed a relevant commit; This should cover most, if not all, by-value fallible conversion between primitives, BigInt and BigUint.

[cuviper]

I added a couple tweaks for simplification, but otherwise it looks good, thanks!

bors r+

[bors]

Build succeeded

• continuous-integration/travis-ci/push