Check for BinaryValue to allow types to use a different binary encoding

[arp242]

The driver.Valuer is used for both text and binary representation, which isn't necessarily desired as the binary representation can be different.

So also check for "BinaryValue() ([]byte, error)" in CheckNamedValue(), and use that if it exists.

For the time being this is intentionally unexported and undocumented. I think this should be right, but I want to implement some more binary types before I'm sure.

Also make the Hstore.Value() a bit faster:

goos: linux
goarch: amd64
pkg: github.com/lib/pq/hstore
cpu: AMD Ryzen 7 PRO 7840U w/ Radeon 780M Graphics

BenchmarkHstore/Value-16                 1228444               979.5 ns/op           904 B/op         11 allocs/op
BenchmarkHstore/BinaryValue-16           3443780               362.9 ns/op          1008 B/op          3 allocs/op

Previous for Value:
BenchmarkHstore/Value-16                  514468              2084 ns/op            2288 B/op         46 allocs/op

Could make it faster still, but this was quick, fast, and obvious to do, and gets us more than double the performance.

Fixes #705