A pos value represents a file-set specific, accurate
source position value. It is 8x smaller in size than
the corresponding Position value (4 bytes vs 32 bytes).

Using Pos values instead of Position values in AST
saves approx. 25MBytes of memory when running godoc
on the current repository.

This CL introduces the Pos, File, and FileSet data
types; it does not affect existing code. Another
(pending CL) will make the change to all dependent
source files.

Missing: tests

R=r
CC=golang-dev, rsc
https://golang.org/cl/2936041