runtime: allocate maps' first bucket table lazily

Motivated by garbage profiling in HTTP benchmarks. This
changes means new empty maps are just one small allocation
(the HMap) instead the HMap + the relatively larger h->buckets
allocation. This helps maps which remain empty throughout
their life.

benchmark               old ns/op    new ns/op    delta
BenchmarkNewEmptyMap          196          107  -45.41%

benchmark              old allocs   new allocs    delta
BenchmarkNewEmptyMap            2            1  -50.00%

benchmark               old bytes    new bytes    delta
BenchmarkNewEmptyMap          195           50  -74.36%

R=khr, golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7722046
cmd/vet: fix assembly test files
They should be build-tagged for vet_test not ignore,
and not have a Go package clause.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8016047
vet: fix test_deadcode.go's tests
This is a totally mechanical change.
Errors are reported for the beginning of the statement, not the end,
so the errchk markers need to be on the opening brace, not the closing
one. It seems this test was never run.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7746050
text/template: fix bug in evaluating a chain starting with a function.

R=golang-dev, alberto.garcia.hierro
CC=golang-dev
https://codereview.appspot.com/7861046