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...