Update README to remove nightly recommendation.

The preceding `cargo publish` was done starting from the wrong local
state; I had an inconsistent state relative to GitHub because of locally
testing the fuzz targets PR.  Oops.

Harden stable implementation of black_box

outdated fuzzers code

Optimize black_box

Add caching for cargo dependencies in Travis builds.

travis: Fix no-std build

b43b139d effectively disables the branches produced by LLVM, so
remove the statement that the non-`nightly` `black_box` does not
protect against these optimizations.

In Rust, `core::ptr::read_volatile` is not formally specified.
However the current documentation mentions that "the semantics
will almost always end up pretty similar to C11's definition of
volatile".

In C11, a volatile read *always* reads a value from memory. So this
value *must* be in RAM in that form, at some time.

This allows us to create a relatively efficient optimization
barrier, by spilling the value to the stack, and immediadiately
reloading it again.

The benefit is that LTO could ever optimize the empty function
away, even if #[inline(always)] is specified (although this may
be very unlikely). However, `volatile` is concretely specified,
so this optimization barrier will not be lost.

Add `or_else` method to `CtOption`

'crypto' means 'cryptography'

New shorter, more efficient ct_eq() for integers.

The previous assembly code required the compiler to move the value into memory to be able to pass the pointer into assembly. Often this also resulted in the function not being inlined. I believe this code is equivalent in terms of black box behavior but allows for much better codegen in the case where `input` is already in a register.

Implement ConditionallySelectable for Choice