Add fuzzing support

[YaLTeR]

Find assertion fails in your own rav1e clone in one simple command.

Closes #115, although new issues could be opened for adding, for example, proptest, or the randomizing RDO decisions thing.

The design I chose here (putting actual fuzzable functions inside the rav1e crate) should make it easy to hook up other fuzzers such as AFL and honggfuzz (more fuzzers more better), although it doesn't make it easy to hook up proptest or quickcheck because those frameworks want you to generate each individual parameter through them, rather than feeding in a chunk of random bytes (this is so that they can meaningfully shrink and expand the inputs, which is the whole point).

Additionally included is the ability to inspect crashing state straight through the fuzz target executables via the RUST_LOG=debug environment variable, which is extremely handy for figuring out what's up and making test cases.

Thus far trophies include everything fixed in #1619 and #1630 and a more-interesting case for #1611.

[coveralls]

Coverage remained the same at 70.965% when pulling 088153f on YaLTeR:fuzzing into cc170c2 on xiph:master.

[lu-zero]

Looks pretty and not too intrusive.

If you switch to the log crate to avoid the explicit debug variable it should get even more compact.

[YaLTeR]

Changed to use debug!().

[lu-zero]

Seems still good :)