runtime: add spanSet data structure

This change copies the gcSweepBuf data structure into a new file and
renames it spanSet. It will serve as the basis for a heavily modified
version of the gcSweepBuf data structure for the new mcentral
implementation.

We move it into a separate file now for two reasons:
1. We will need both implementations as they will coexist simultaneously
   for a time.
2. By creating it now in a new change it'll make future changes which
   modify it easier to review (rather than introducing the new file then).

Updates #37487.

Change-Id: If80603cab6e813a1ee2e5ecd49dcde5d8045a6c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/221179


Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>