golang/x/perf/benchlib: New module to load & work with data in Go benchmark format

Go benchmark format is described here:

	https://github.com/golang/proposal/blob/master/design/14313-benchmark-format.md

Additionally we support extension to that format:

	- a line starting with `*** neotest:` denotes start of neotest extension block.
	  The block consists of labels describing e.g. hardware and software on a node.
	  The block ends with a blank line.
	  Labels in the block are not added to benchmarking lines from main stream.
	  The block itself should not contain benchmark lines.

and upon processing benchmark units are normalized to common base,
similarly to

	golang.org/cl/82955
	golang.org/cl/82956
	golang.org/cl/82957

Orignally implemented here:

	kirr/neo@502d9477
	kirr/neo@a9b10a45
	kirr/neo@f5fec740
	kirr/neo@916782b6