Improve Rust bindings and build tools

[michaelsproul]

This PR incorporates two changes that have been lurking in sigp's fork of BLST that should really live upstream:

• Implement the Eq trait for PublicKey and Signature (resolves Rust bindings: add typical impls #12)
• Tweak build.rs so it supports the two compilation configurations we want via Cargo features:
  • feature = portable: for pre-2013 CPUs, disables ADX, turns on __BLST_PORTABLE__ in the C build
  • feature = force-adx: for post-2013 CPUs, enables ADX even if it isn't detected on the host
  • feature = default (no features provided): for all CPUs and source builds, enables ADX if it is present on the host

We're likely to ship two binaries, one portable and one force-adx. The reason for making force-adx its own feature is so we can know when a binary is intended for a modern CPU, and raise an appropriate runtime error if ADX support isn't detected at runtime (instead of just SIGILL).

I also replaced is_adx by a call to the is_x86_feature_detected! macro from the standard library, which is equivalent as far as I understand.

[dot-asm]

Merged. Thanks!