A co creation with bradfitz

* add fast path for header lines which are not continuations
* pass hint to better size initial mime header map

lucky(~/go/src/pkg/net/http) % ~/go/misc/benchcmp {golden,new}.txt
benchmark                          old ns/op    new ns/op    delta
BenchmarkReadRequestChrome             10073         8348  -17.12%
BenchmarkReadRequestCurl                4368         4350   -0.41%
BenchmarkReadRequestApachebench         4412         4397   -0.34%
BenchmarkReadRequestSiege               6431         5924   -7.88%
BenchmarkReadRequestWrk                 2820         3146  +11.56%

benchmark                           old MB/s     new MB/s  speedup
BenchmarkReadRequestChrome             60.66        73.18    1.21x
BenchmarkReadRequestCurl               17.85        17.93    1.00x
BenchmarkReadRequestApachebench        18.58        18.65    1.00x
BenchmarkReadRequestSiege              23.48        25.49    1.09x
BenchmarkReadRequestWrk                14.18        12.71    0.90x

benchmark                         old allocs   new allocs    delta
BenchmarkReadRequestChrome                32           26  -18.75%
BenchmarkReadRequestCurl                  15           15    0.00%
BenchmarkReadRequestApachebench           16           15   -6.25%
BenchmarkReadRequestSiege                 22           19  -13.64%
BenchmarkReadRequestWrk                   11           11    0.00%

benchmark                          old bytes    new bytes    delta
BenchmarkReadRequestChrome              3148         2216  -29.61%
BenchmarkReadRequestCurl                 905         1413   56.13%
BenchmarkReadRequestApachebench          956         1413   47.80%
BenchmarkReadRequestSiege               1397         1522    8.95%
BenchmarkReadRequestWrk                  757         1369   80.85%

R=bradfitz
CC=golang-dev
https://golang.org/cl/7300098