Simplify core vs std usage

[josephlr]

Right now, src/lib.rs defines ring to be a no_std crate conditionally based on the features and CPU architecture. This is sub-optimal, as it requires a lot of cfg attributes to be kept in sync. Both rand and rdrand have dealt with this kind of problem, and they use the following solution.

• The create is unconditionally #![no_std]
• If std is needed, it can be brought in with extern crate std
• There are no bare use std or use core statements (as they are never necessary)

This is what I did in this PR. Some advantages to this PR:

• It is now very easy to see what ring is depending on from std. This helps with:
  • Document how to port *ring* to a no_std-only platform #744
  • Eventually making ring's tests work with no_std
• After extern crate alloc lands in the next stable Rust release, most of the use std:: statements can be turned into use alloc::.
• It will make it easier to eventually split the use_heap feature into two features
  • std: Enables getting randomness from files and std::error::Error implementations
  • alloc: Enables operations that need dynamic memory allocation (RSA signing, etc...)
• Helps with the porting work in Add support for x86_64-unknown-uefi #787 and Port to SGX #775

[briansmith]

Thanks!