go/token: use fine-grained locking in FileSet

go/token: use fine-grained locking in FileSet

Before, all accesses to the lines and infos tables of each File were
serialized by the lock of the owning FileSet, causing parsers running
in parallel to contend.  Now, each File has its own mutex.

This fixes a data race in (*File).PositionFor, which used to call
f.position then f.unpack without holding the mutex's lock.

Fixes golang/go#18348

Change-Id: Iaa5989b2eba88a7fb2e91c1a0a8bc1e7f6497f2b
Reviewed-on: https://go-review.googlesource.com/34591
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>