Stop using "Int" for microsecond delays in "base"

The "base" library has the "threadDelay" primitive, which takes an Int argument in microseconds. Unfortunately this means that the longest delay you can get on a 32 bit machine with GHC is just under 36 minutes (2^31 uSec), and a hypothetical compiler that only used 30 bit integers (as per the standard) would get under 10 minutes. It is a bit tricky to write general-purpose libraries with this. I think that there should be a type Delay = Int64 declaration, and that threadDelay and related functions should take that as an argument type. Paul.