Use LZ4 or Snappy instead of Deflate in metadata

[emberian]

Currently decompressing the metadata is ~10% (more on slower computers) of the build time of small crates (eg, the tests). Switching to a faster algorithm would bring that down. I think LZ4 is that algorithm:

• Compatibly licensed implementation (I think. @graydon? It's 2-clause BSD)
• Almost as good a compression ratio as zlib
• 5x faster decompression than zlib
• Much faster than snappy, marginally worse compression.

This has two parts:

• Add LZ4 wrapper
• Use it in rustc::metadata

[emberian]

I don't know how bootstrapping this change is going to work, either.

[graydon]

I think the eventual ideal move here is to compress each item's metadata and ast separately and fix how we load metadata to only decompress the necessary items. But picking a different compressor, or different encoding, is certainly also fine.

[emberian]

Yeah, transitioning away from ebml would be great. Decoding it and especially converting from big endian is a sore spot. Of course, reading as little endian right now isn't any better because it still does conversion and doesn't optimize for little endian systems, but that optimization is at least possible.

[emberian]

Alright so given the numbers #6954, I don't think LZ4 is worth it, and neither does @brson.