cmd/go: add coverage analysis

cmd/go: add coverage analysis
This feature is not yet ready for real use. The CL marks a bite-sized
piece that is ready for review. TODOs that remain:
        provide control over output
        produce output without setting -v
        make work on reflect, sync and time packages
                (fail now due to link errors caused by inlining)
        better documentation
Almost all packages work now, though, if clumsily; try:
        go test -v -cover=count encoding/binary

R=rsc
CC=gobot, golang-dev, remyoudompheng
https://golang.org/cl/10050045