go/token: faster FileSet.Position implementation

- added a cache for last file looked up: avoids binary
  search if the file matches

- don't look up extra line info if not present
  (it is almost never present)

- inline one critical binary search call (inlining
  provides almost 30% improvement in this case)

Together, these changes make the go/printer benchmark
more than twice as fast (53% improvement). gofmt also
sped up by about the same amount.


Also: removed an unused internal field from FileSet.

Measurements (always best of 5 runs):

* original:
  printer.BenchmarkPrint     5    238354200 ns/op (100%)

* using last file cache:
  printer.BenchmarkPrint    10    201796600 ns/op (85%)

* avoiding lookup of extra line info:
  printer.BenchmarkPrint    10    157072700 ns/op (66%)

* inlining a critical binary search call:
  printer.BenchmarkPrint    10    111523500 ns/op (47%)

gofmt (always best of 3 runs):

* before:
  time gofmt -l src misc
  real	0m33.316s
  user	0m31.298s
  sys	0m0.319s

* after:
  time gofmt -l src misc
  real	0m15.889s
  user	0m14.596s
  sys	0m0.224s

R=r, dfc, bradfitz, rsc1
CC=golang-dev
http://codereview.appspot.com/4433086

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