Global string intern pool

Internal use of DashMap for concurrent support

Examples

IStr

use pstr::IStr;
let s = IStr::new("hello world");

MowStr

use pstr::MowStr;
let mut s = MowStr::new("hello");
assert!(s.is_interned());

s.push_str(" ");
assert!(s.is_mutable());

s.mutdown().push_str("world");
assert_eq!(s, "hello world");

s.intern();
assert!(s.is_interned());