More tests and examples

[mberry]

• Add: Basic example with message encryption
• Modify: constant visibility
• Removed: benchmarking feature, criterion now back in dev-deps
• Add: multiple KEX tests in various failure modes
• Update: docs and readme closes #55, closes #53

[mberry]

Criterion as a dev-dep is causing ASAN checks to fail in the test suite. Reverted back to it being behind the benchmarking features

[mberry]

AEAD as an dev-dependency for the example is failing on rust 1.47 because of its MSRV

[mberry]

Removed full hybrid PKE example. Due to interactions with dev-deps, CI and MSRV it's simpler to move the example elsewhere. The CI actions could be refactored to accommodate them but pulling in additional dev-deps just for an example are a cold-compile burden on maintainers, with all the feature combinations in testing this adds up. A separate repo containing examples seems more prudent long-term. The benchmarking feature should eventually go into RUSTFLAGS just like the KAT's did but I've left that for now. No breaking changes made.