Use the `jpeg-encoder` crate for a 2x speedup

[Shnatsel]

The jpeg-encoder crate implements a pure-Rust JPEG encoder, with unsafe only used for SIMD.

In my tests encoding a large image with it is over 2x faster than the current encoder (332 ms vs the current 785ms on x86).

Correctness seems good too - there were no issues on my web-scraped JPEG corpus (23,000 images). The crate also seems to be extensively fuzzed.