util: add a stream compression API to compression engines It is a common pattern throughout the code to perform compression on an iterator of chunks, yielding an iterator of compressed chunks. Let's formalize that as part of the compression engine API. The zlib and bzip2 implementations allow an optional "level" option to control the compression level. The default values are the same as what the Python modules use. This option will be used in subsequent patches.

util: remove decompressors dict (API) All in-tree consumers are now using the compengines registrar. Extensions should switch to it as well.

changegroup: use compression engines API The new API doesn't have the equivalence for None and 'UN' so we introduce code to use 'UN' explicitly.