go/token: implement PositionFor accessors

Package addition.

PositionFor permits access to both, positions
adjusted by //line comments (like the Position
accessors), and unadjusted "raw" positions
unaffected by //line comments.

Raw positions are required for correct formatting
of source code via go/printer which until now had
to manually fix adjusted positions.

Fixes issue 7702.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/135110044