Global string intern pool

Internal use of DashMap for concurrent support

Examples

• IStr rust use pstr::IStr; let s = IStr::new("hello world");
• MowStr ```rust use pstr::MowStr; let mut s = MowStr::new("hello"); assert!(s.is_interned());

s.pushstr(" "); assert!(s.ismutable());

s.mutdown().pushstr("world"); asserteq!(s, "hello world");

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