integer-simple by default

[Greg Fitzgerald]

> You can dynamically link libgmp on windows. That might be easier:

Do you know if the dynamic link escape hatch has ever held up in
court?  Last time I looked into it, the free software community had
mixed opinions.  In any case, giving GMP the boot alleviates any
licensing concerns, makes the GHC build a little simpler, and allows
users to create standalone executables.  Is there any reason we
shouldn't attempt to make integer-simple the default?

-Greg