Add string.startswith and string.endswith

[s-hadinger]

See @405. Add:

• string.startswith(hay:string, needle:string [, case_insensitive:bool]) -> bool
• string.endswith(hay:string, needle:string [, case_insensitive:bool]) -> bool

These new methods are lightweight, allow for case sensitive or case insensitive matches, and don't need to allocate any temporary object.