Allow fiat_backend and simd_backend to be activated at the same time

[tarcieri]

Right now fiat_backend and simd_backend are mutually exclusive: activating both crate features generates a compile error.

In #414 we discussed various alternatives including something like --cfg dalek_backend="simd". However, since the backends map to optional crate dependencies, features seem like a more natural fit.

As an alternative to a compile error, another option would be to handle the case of both features being activated by choosing a "winner" backend which is selected when both options are enabled.

In #414 I suggested that given various tradeoffs, it should probably be fiat_backend because it would be bad to enable a non-verified backend when someone has asked for a formally verified one (even though simd_backend is superior in pretty much all other respects).

This isn't a blocker for a v4.0 release though, as it can easily be added afterward as it changes a case which is currently a compile error into one that works.

[pinkforest]

PR's up:
- #463

It's blocked by:
- #464 EDIT: nm: this was already resolved by #455

[pinkforest]

So the above PR was my confusion - This was really resolved by #455 - #463 (comment)

This issue should be closed probably.