readContinuedLineSlice intends to buffer a continued line of text, where
a continued line can continue through newlines so long as the next line
begins with a space or tab.

The current optimization is to not try to buffer and build a line if we
immediately see that the next line begins with an ASCII character.

This adds avoiding copying the line if we see that the next line is \n
or \r\n as well.

Notably, headers always end in \r\n\r\n. In the general, well formatted
header case, we can now avoid ever allocating textproto.Reader's
internal reusable buf.

This can mildly be seen in net/http's BenchmarkClientServer:

name            old time/op    new time/op    delta
ClientServer-4    66.4µs ± 0%    66.2µs ± 0%  -0.35%  (p=0.004 n=10+10)

name            old alloc/op   new alloc/op   delta
ClientServer-4    4.87kB ± 0%    4.82kB ± 0%  -1.01%  (p=0.000 n=6+10)

name            old allocs/op  new allocs/op  delta
ClientServer-4      64.0 ± 0%      63.0 ± 0%  -1.56%  (p=0.000 n=10+10)

Change-Id: Id8c2ab69086ac481b90abda289396dcb7bfe8851
Reviewed-on: https://go-review.googlesource.com/c/134227Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>