pgtype/hstore: Make text parsing about 6X faster

I am working on an application that uses hstore types, and we found
that returning the values is slow, particularly when using the text
protocol, such as when using database/sql. This improves parsing to
be about 6X faster (currently faster than binary). The changes are:

* referencing the original string instead of copying into new strings
  (very large win)
* using string.IndexByte to scan double quoted strings: it has
  architecture-specific assembly implementations, and most of the
  time is spent in key/value strings.
* estimating the number of key/value pairs to allocate the correct
  size of the slice and map up front. This reduces the number of
  allocations and bytes allocated by a factor of 2, and was a small
  CPU win.
* parsing directly into the Hstore, rather than copying into it.

This parser is stricter than the old one. It only accepts hstore
strings serialized by Postgres. The old one was already stricter
than Postgres's own parser, but previously accepted any whitespace
character after a comma. This one only accepts space. Example:

  "k1"=>"v1",\t"k2"=>"v2"

Postgres only ever uses ", " as the separator. See hstore_out:
https://github.com/postgres/postgres/blob/master/contrib/hstore/hstore_io.c

The result of using benchstat to compare the benchmark on my M1 Pro
with the following command line in below. The new text parser is now
faster than the binary parser. I will improve the binary parser in a
separate change.

for i in $(seq 10); do go test ./pgtype -run=none -bench=BenchmarkHstoreScan -benchtime=1s >> new.txt; done

goos: darwin
goarch: arm64
pkg: github.com/jackc/pgx/v5/pgtype
                               │  orig.txt   │               new.txt               │
                               │   sec/op    │   sec/op     vs base                │
HstoreScan/databasesql.Scan-10   82.11µ ± 1%   10.51µ ± 0%  -87.20% (p=0.000 n=10)
HstoreScan/text-10               83.30µ ± 1%   11.49µ ± 1%  -86.20% (p=0.000 n=10)
HstoreScan/binary-10             15.99µ ± 2%   15.77µ ± 1%   -1.35% (p=0.007 n=10)
geomean                          47.82µ        12.40µ       -74.08%

                               │   orig.txt   │               new.txt                │
                               │     B/op     │     B/op      vs base                │
HstoreScan/databasesql.Scan-10   56.23Ki ± 0%   11.68Ki ± 0%  -79.23% (p=0.000 n=10)
HstoreScan/text-10               65.12Ki ± 0%   20.58Ki ± 0%  -68.40% (p=0.000 n=10)
HstoreScan/binary-10             21.09Ki ± 0%   21.09Ki ± 0%        ~ (p=0.378 n=10)
geomean                          42.58Ki        17.18Ki       -59.66%

                               │  orig.txt   │               new.txt                │
                               │  allocs/op  │ allocs/op   vs base                  │
HstoreScan/databasesql.Scan-10   744.00 ± 0%   44.00 ± 0%  -94.09% (p=0.000 n=10)
HstoreScan/text-10               743.00 ± 0%   44.00 ± 0%  -94.08% (p=0.000 n=10)
HstoreScan/binary-10              464.0 ± 0%   464.0 ± 0%        ~ (p=1.000 n=10) ¹
geomean                           635.4        96.49       -84.81%
¹ all samples are equal