Support generics

[kasegao]

Added generics for all levels of OrderedMap implementation.
For backward compatibility, generics is only enabled on go 1.18 and later versions.

Most structures and functions can be called with the original name with a suffix "G". For example, "OrderMapG" is a generics version of "OrderMap".

Here are the results of my benchmark tests:

> go test -bench . -benchmem
goos: windows
goarch: amd64
pkg: github.com/cevaris/ordered_map
cpu: Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz
BenchmarkSet_OrderedMap-12               1350037               759.2 ns/op           261 B/op          3 allocs/op
BenchmarkSet_OrderedMapG-12              5101459               248.8 ns/op            99 B/op          1 allocs/op
BenchmarkGet_OrderedMap-12              15139624                93.70 ns/op            0 B/op          0 allocs/op
BenchmarkGet_OrderedMapG-12             21065011                76.77 ns/op            0 B/op          0 allocs/op
BenchmarkDelete_OrderedMap-12            3796270               332.9 ns/op             0 B/op          0 allocs/op
BenchmarkDelete_OrderedMapG-12          10951123               128.5 ns/op             0 B/op          0 allocs/op
PASS
ok      github.com/cevaris/ordered_map  45.006s

[cevaris]

go test -bench . -benchmem
can we also add a new Makefile target, and update the README.md to say how to run benchmarks; ie. go bench.

bench:
	go test -bench . -benchmem

[cevaris]

To confirm, the original non-generic code cannot be reused for generic code? If so, that would be ideal?

[eugbyte]

Please approve and merge this PR, it would be very helpful