util: remove compressorobj API from compression engines All callers have been replaced with "compressstream." It is quite low-level and redundant with "compressstream." So eliminate it.

hgweb: use compression engine API for zlib compression More low-level compression code elimination because we now have nice APIs. This patch also demonstrates why we needed and implemented the "level" option on the "compressstream" API.

bundle2: use compressstream compression engine API Compression engines now have an API for compressing a stream of chunks. Switch to it and make low-level compression code disappear.

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.

bundle2: use compression engines API to obtain decompressor Like the recent change for the compressor side, this too is relatively straightforward. We now store a compression engine on the instance instead of a low-level decompressor. Again, this will allow us to easily transition to different compression engine APIs when they are implemented.

util: remove compressors dict (API) We no longer have any in-tree consumers of this object. Use util.compengines instead.

bundle2: use new compression engine API for compression Now that we have a new API to define compression engines, let's put it to use! The new code stores a reference to the compression engine instead of a low-level compressor object. This will allow us to more easily transition to different APIs on the compression engine interface once we implement them. As part of this, we change the registration in bundletypes to use 'UN' instead of None. Previously, util.compressors had the no-op compressor registered under both the 'UN' and None keys. Since we're switching to a new API, I don't see the point in carrying this dual registration forward.

util: create new abstraction for compression engines Currently, util.py has "compressors" and "decompressors" dicts mapping compression algorithms to callables returning objects that perform well-defined operations. In addition, revlog.py has code for calling into a compressor or decompressor explicitly. And, there is code in the wire protocol for performing zlib compression. The 3rd party lz4revlog extension has demonstrated the utility of supporting alternative compression formats for revlog storage. But it stops short of supporting lz4 for bundles and the wire protocol. There are also plans to support zstd as a general compression replacement. So, there appears to be a market for a unified API for registering compression engines. This commit starts the process of establishing one. This commit establishes a base class/interface for defining compression engines and how they will be used. A collection class to hold references to registered compression engines has also been introduced. The built-in zlib, bz2, truncated bz2, and no-op compression engines are registered with a singleton instance of the collection class. The compression engine API will change once consumers are ported to the new API and some common patterns can be simplified at the engine API level. So don't get too attached to the API...