immutability: make string types immutable.
immutability: make BigInt and BigFloat immutable.

This is slightly tricky since it requires moving the finalizer to the
data array inside of each of these, rather than the object itself,
but technically, that's more correct anyway, since if by some weird
circumstance multiple BigInts shared the same data array, the
finalizer should only be called on that data once.
immutability: make ranges immutable.
immutability: make regex-related types immutable.
immutability: make Stat immutable.
immutability: make sort Orderings immutable.
Merge branch 'master' of github.com:JuliaLang/julia