plumbing: packfile/scanner, readability/performance improvements, zlib pooling

[saracen]

This migrates scanner's trackableReader, byteReadSeeker and teeReader's functionality into one scannerReader. I wanted the ability to have a pool of Scanners if need be, and offering a Reset() and dealing with all 3 readers was proving difficult. I believe this version is also easier to read.

In addition:

• There's a zlib pool. Pooling zlib isn't obvious, as although it has a Reset() function, zlib.NewReader doesn't accept nil. This limitation is by-passed by priming the reader with a zero byte compressed payload. I couldn't find a better way of doing this.
• Previously, writing to the crc hash still often performed small writes (via Read(), not just ReadByte()). Now all small reads are buffered when they need to be. The buffer size is now smaller and sized to take advantage of the CLMUL instruction crc32 uses.
• Deprecated scanner's Flush(). There's no need for this to be public.

[filipnavara]

I have seen some woefully unoptimal code paths in that code when I was recently profiling Gitea. I'd gladly give it a try once it passes the CI tests. It may be a better alternative to #1122.

[jfontan]

It would be nice to hace a test that reuses scanner with Reset. Other than that LGTM! Thanks!

[saracen]

Good shout! I noticed a problem whilst figuring out how/what to test. Even when the reader is seekable, I'm returning the r.offset when whence == io.SeekCurrent && offset == 0, that incorrectly assumes that the reader provided to start with was set at the beginning of the file.

I'll fix that and add a test shortly!

[saracen]

I've added two tests. TestReaderReset checks basics usage and that fields are correctly reset. TestReaderResetSeeks checks that the seek behavior is correct.