librustc: Apply null pointer optimization to slices, closures and trait
objects.
librustc: Remove match arm since we don't allow enum to float casts.
librustc: Don't create &[T] slices with NULL as the ptr.
Add tests for null pointer opt.
auto merge of #15406 : luqmana/rust/nop, r=pcwalton

Extend the null ptr optimization to work with slices, closures, procs, & trait objects by using the internal pointers as the discriminant.

This decreases the size of `Option<&[int]>` (and similar) by one word.