Rewrite using generics, Go Module support

• The package is now a go module and can be used as a dependency using
  go get github.com/m-murad/ordered-sync-map.
• The whole package has been rewritten using generics. You can declare the type of keys and variables for the map.

import (
	osm "github.com/m-murad/ordered-sync-map"
)

func main() {
	mp := osm.New[string, string]()
}