internal/coverage: add coverage meta-data encoder

internal/coverage: add coverage meta-data encoder

Add a new package with APIs for encoding coverage meta-data. This
provides support for accumulating information about each function
during the compilation process, and then encoding and emitting a
payload for a coverage meta-data symbol.  Not yet connected to the
rest of the coverage machinery (that will appear in a later patch).

Updates #51430.

Change-Id: I61054ce87f205b25fb1bfedaa740fd7425c34de4
Reviewed-on: https://go-review.googlesource.com/c/go/+/353453
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>