Make BitSlice's `Word` properly generic.

Currently `Word` is `usize`, and there are various places in the code
that assume this.

This patch mostly just changes `usize` occurrences to `Word`. Most of
the changes were found as compile errors when I changed `Word` to a type
other than `usize`, but there was one non-obvious case in
librustc_mir/dataflow/mod.rs that caused bounds check failures before I
fixed it.

0 files changed, 0 insertions, 0 deletions