encoding/csv: simplify and optimize Reader

The Reader implementation is slow because it operates on a rune-by-rune
basis via bufio.Reader.ReadRune. We speed this up by operating on entire
lines that we read from bufio.Reader.ReadSlice.

In order to ensure that we read the full line, we augment ReadSlice
in our Reader.readLine method to automatically expand the slice if
bufio.ErrBufferFull is every hit.

This change happens to fix #19410 because it no longer relies on
rune-by-rune parsing and only searches for the relevant delimiter rune.

In order to keep column accounting simple and consistent, this change
reverts parts of CL 52830.

This CL is an alternative to CL 36270 and builds on some of the ideas
from that change by Diogo Pinela.

name                                     old time/op    new time/op    delta
Read-8                                   3.12µs ± 1%    2.54µs ± 2%  -18.76%   (p=0.000 n=10+9)
ReadWithFieldsPerRecord-8                3.12µs ± 1%    2.53µs ± 1%  -18.91%    (p=0.000 n=9+9)
ReadWithoutFieldsPerRecord-8             3.13µs ± 0%    2.57µs ± 3%  -18.07%  (p=0.000 n=10+10)
ReadLargeFields-8                        52.3µs ± 1%     5.3µs ± 2%  -89.93%   (p=0.000 n=10+9)
ReadReuseRecord-8                        2.05µs ± 1%    1.40µs ± 1%  -31.48%   (p=0.000 n=10+9)
ReadReuseRecordWithFieldsPerRecord-8     2.05µs ± 1%    1.41µs ± 0%  -31.03%   (p=0.000 n=10+9)
ReadReuseRecordWithoutFieldsPerRecord-8  2.06µs ± 1%    1.40µs ± 1%  -31.70%   (p=0.000 n=9+10)
ReadReuseRecordLargeFields-8             50.9µs ± 0%     4.1µs ± 3%  -92.01%  (p=0.000 n=10+10)

name                                     old alloc/op   new alloc/op
Read-8                                       664B ± 0%      664B ± 0%
ReadWithFieldsPerRecord-8                    664B ± 0%      664B ± 0%
ReadWithoutFieldsPerRecord-8                 664B ± 0%      664B ± 0%
ReadLargeFields-8                          3.94kB ± 0%    3.94kB ± 0%
ReadReuseRecord-8                           24.0B ± 0%     24.0B ± 0%
ReadReuseRecordWithFieldsPerRecord-8        24.0B ± 0%     24.0B ± 0%
ReadReuseRecordWithoutFieldsPerRecord-8     24.0B ± 0%     24.0B ± 0%
ReadReuseRecordLargeFields-8               2.98kB ± 0%    2.98kB ± 0%

name                                     old allocs/op  new allocs/op
Read-8                                       18.0 ± 0%      18.0 ± 0%
ReadWithFieldsPerRecord-8                    18.0 ± 0%      18.0 ± 0%
ReadWithoutFieldsPerRecord-8                 18.0 ± 0%      18.0 ± 0%
ReadLargeFields-8                            24.0 ± 0%      24.0 ± 0%
ReadReuseRecord-8                            8.00 ± 0%      8.00 ± 0%
ReadReuseRecordWithFieldsPerRecord-8         8.00 ± 0%      8.00 ± 0%
ReadReuseRecordWithoutFieldsPerRecord-8      8.00 ± 0%      8.00 ± 0%
ReadReuseRecordLargeFields-8                 12.0 ± 0%      12.0 ± 0%

Updates #22352
Updates #19019
Fixes #16791
Fixes #19410

Change-Id: I31c27cfcc56880e6abac262f36c947179b550bbf
Reviewed-on: https://go-review.googlesource.com/72150
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>