The conversion was done with c2rust and then manually cleaned up some.
There's still lots of unsafe code remaining but I'd rather do the rest
of the cleanup in-tree so that it's easier to catch and revert
regressions.
I've dropped support for SSE1 (Firefox requires SSE2) and Altivec
(not wanting to deal with fixing Rust's Altivec support).
transform_neon.rs manually implements a bunch of intrinsics, this
can be fixed when we can depend on Rust 1.48.
The fuzz target is changed to a cargo-fuzz target.
Some of the gtests are moved to Rust.
Differential Revision: https://phabricator.services.mozilla.com/D90782