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;letmut 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());