revert PriorityQueue to using init()

uninit() would result in potentially running a destructor on arbitrary
memory if the Ord implementation throws
Remove re-exports from libcore/core.rc

Also fix up all the fallout elsewhere throughout core. It's really nice being
able to have the prelude.
Add inverse hyperbolic functions
Add ldexp and frexp functions
Remove unnecessary infinity check
Cleanup vec::len(var) to var.len()