go/ast: use token.Pos instead of token.Position; adjust all dependent code

	go/ast: use token.Pos instead of token.Position; adjust all dependent code

	Specifically:

	* lib/godoc:
	- provide file set (FSet) argument to formatters where needed

	* src/cmd:
	- cgo, ebnflint, godoc, gofmt, goinstall: provide file set (fset) where needed
	- godoc: remove local binary search with sort.Search (change by rsc),
	  extract file set for formatters

	* src/pkg:
	- exp/eval: remove embedded token.Position fields from nodes and replace
	  with named token.Pos fields; add corresponding Pos() accessor methods
	- go/token: added file.Line(), changed signature of File.Position()

	* test/fixedbugs/:
	- bug206.go: change test to not rely on token.Pos details

	* added various extra comments
	* Runs all.bash
	* gofmt formats all of src, misc w/o changes
	* godoc runs

	* performance:
	- The new version of godoc consumes about the same space after indexing
	  has completed, but indexing is half the speed. Significant space savings
	  are expected from smaller ASTs, but since they are thrown away after a
	  file has been indexed, this is not visible anymore. The slower indexing
	  time is due to the much more expensive computation of line information.
	  However, with the new compressed position information, indexing can be
	  rewritten and simplified. Furthermore, computing the line info can be
	  done more efficiently.

        New godoc, immediately after indexing completed (best of three runs):

	  PID COMMAND      %CPU   TIME   #TH #PRTS #MREGS RPRVT  RSHRD  RSIZE  VSIZE
	44381 godoc        0.0%  0:38.00   4    19    149  145M   184K   148M   176M

	2010/12/03 17:58:35 index updated (39.231s, 18505 unique words, 386387 spots)
	2010/12/03 17:58:35 bytes=90858456 footprint=199182584
	2010/12/03 17:58:36 bytes=47858568 footprint=167295224

	Old godoc, immediately after indexing completed (best of three runs):

	  PID COMMAND      %CPU   TIME   #TH #PRTS #MREGS RPRVT  RSHRD  RSIZE  VSIZE
	23167 godoc        0.0%  0:22.02   4    17    132  129M   184K   132M   173M

	2010/12/03 14:51:32 index updated (24.892s, 18765 unique words, 393830 spots)
	2010/12/03 14:51:32 bytes=66404528 footprint=163907832
	2010/12/03 14:51:32 bytes=46282224 footprint=163907832

	The different numbers for unique words/spots stem from the fact the the
	two workspaces are not exactly identical. The new godoc maintains a large
	file set data structure during indexing which (probably) is the reason
	for the larger heap (90858456 vs 66404528) before garbage collection.

R=rsc, r
CC=golang-dev
https://golang.org/cl/3050041