CI: more comprehensive, cross compile to big-endian, faster (+ criterion, docs.rs and 1.20 minimum rustc version)

[huonw]

Commit summary:

• docs.rs is reliable enough that there's less need to publish docs manually
• https://github.com/japaric/cross makes it simple to get CI on unusual architectures, such as mips (which is an "allowed_failure" currently: #10)
• the default cargo test --all is now 15-20x faster by using smaller inputs for slow tests (or disabling them entirely, if there's no small input that makes sense). --features "primal-sieve/slow_tests primal-slowsieve/slow_tests" reenables the full tests.
• criterion allows for benchmarking on stable
• the minimum rustc version moves to 1.20.0 because smallvec needs it: https://travis-ci.org/huonw/primal/jobs/463339021

[autohuonw]

@huonw: no appropriate reviewer found, use r? to override

[huonw]

bors-ng looks good; I'll investigate setting it up over the next week or so. (I've updated all the Cargo.tomls.)