Compress all interface files generated by the compiler with LZ4. While
being only a tiny amount of code, LZ4 is both fast at compression and
decompression, and has good compression ratios.

Non-scientific size test: size of stage2 compiler .hi files:

`find ./compiler/stage2 -type f -iname '*.hi' -exec du -ch {} + | grep total$`

Without this patch: 22MB of .hi files for stage2.
With this patch: 9.2MB of .hi files for stage2.

Signed-off-by: Austin Seipp <austin@well-typed.com>

Reviewed By: bgamari

Differential Revision: https://phabricator.haskell.org/D1159