Fixed coverage map issues; better aligned with LLVM APIs

Fixed coverage map issues; better aligned with LLVM APIs

Found some problems with the coverage map encoding when testing with
more than one counter per function.

While debugging, I realized some better ways to structure the Rust
implementation of the coverage mapping generator. I refactored somewhat,
resulting in less code overall, expanded coverage of LLVM Coverage Map
capabilities, and much closer alignment with LLVM data structures, APIs,
and naming.

This should be easier to follow and easier to maintain.