math/big: add Rat.{,Set}Float64 methods for IEEE 754 conversions.

Added tests, using input data from strconv.ParseFloat.
Thanks to rsc for most of the test code.

math/big could use some good package-level documentation.

R=remyoudompheng, rsc
CC=golang-dev
https://codereview.appspot.com/6930059
exp/ssa: (#2 of 5): core utilities

This CL includes the implementation of Literal, all the
Value.String and Instruction.String methods, the sanity
checker, and other misc utilities.

R=gri, iant, iant
CC=golang-dev
https://codereview.appspot.com/7199052
exp/ssa: (#3 of 5): Function, BasicBlock and optimisations

R=gri, iant, iant
CC=golang-dev
https://codereview.appspot.com/7202051
exp/gotype: fix build breakage due to https://code.google.com/p/go/source/detail?r=ca5e5de48173

Add 'math/big' to blacklist of packages that use shift
operations as yet unsupported by go/types.

(The failure was masked due to local bugfixes in my client.)

R=rsc, bradfitz, bradfitz
CC=golang-dev
https://codereview.appspot.com/7220057
cmd/ld: avoid a segfault when dumping the symbol table

The dumping routine incorrectly assumed that all incoming
symbols would be non-nil and load through it to retrieve the
symbol name.  Instead of using the symbol to retrieve a name,
use the name provided by the caller.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7224043
test: add test case miscompiled by gccgo

R=golang-dev, bradfitz, rsc, iant
CC=golang-dev
https://codereview.appspot.com/7240043
exp/ssa: fix breakage due to https://code.google.com/p/go/source/detail?r=ca5e5de48173

I don't understand why this didn't show up during my testing.

R=bradfitz
TBR=bradfitz
CC=golang-dev
https://codereview.appspot.com/7237047
encoding/json: add Decoder.Buffered accessor to get overread data

Otherwise it's impossible to know how much data from the
json.Decoder's underlying Reader was actually consumed.

The old fix from golang.org/issue/1955 just added docs. This
provides an actual mechanism.

Update issue 1955

R=golang-dev, adg, rsc
CC=golang-dev
https://codereview.appspot.com/7181053
cmd/api: normalize byte to uint8 and rune to int32

R=golang-dev, adg, mikioh.mikioh
CC=golang-dev
https://codereview.appspot.com/7195049
cmd/gc: document more of the declaration context enumeration

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7231051
go/misc: Adding go-tour to the generated packages for every distribution.

R=adg, minux.ma
CC=golang-dev
https://codereview.appspot.com/6976045
runtime: dump the full stack of a throwing goroutine
Useful for debugging of runtime bugs.
+ Do not print "stack segment boundary" unless GOTRACEBACK>1.
+ Do not traceback system goroutines unless GOTRACEBACK>1.

R=rsc, minux.ma
CC=golang-dev
https://codereview.appspot.com/7098050