Add SRFI-43-compliant vector-copy!.
Add SRFI-43-compliant vector-copy!.

This lets me load microscheme code into racket for testing, which is
super handy! (The SRFI version allows some of the args to be optional,
which we don't support, but I think it's still worth adding.)

I kept the old vector-copy with the non-standard argument order for
backwards-compatibility reasons.

technomancy/microscheme,ryansuchocki/microscheme,ryansuchocki/microscheme,technomancy/microscheme