flif-sys and libflif benchmarks

[newpavlov]

As was discussed in #45.

I've also regenerated sea_snail.flif from png, as existing file was producing the following warning on flif -d:

CORRUPTION DETECTED: checksums don't match (computed: fb34712f v/s read: 6e52a81d)! (partial file?)

Good reason to look into #41?

[newpavlov]

I've also wrote a small CLI tool to check that pure Rust and libflif produce the same decoding result. (it accepts file paths via stdin ad does decoding and comparison in parallel with rayon) I don't think we need it in this repository, but I can add it if you want.