Injecting expressions in place of counters where helpful

Implementing the Graph traits for the BasicCoverageBlock
graph.

optimized replacement of counters with expressions plus new BCB graphviz

* Avoid adding coverage to unreachable blocks.
* Special case for Goto at the end of the body. Make it non-reportable.

Improved debugging and formatting options (from env)

Don't automatically add counters to BCBs without CoverageSpans. They may
still get counters but only if there are dependencies from
other BCBs that have spans, I think.

Make CodeRegions optional for Counters too. It is
possible to inject counters (`llvm.instrprof.increment` intrinsic calls
without corresponding code regions in the coverage map. An expression
can still uses these counter values.

Refactored instrument_coverage.rs -> instrument_coverage/mod.rs, and
then broke up the mod into multiple files.

Compiling with coverage, with the expression optimization, works on
the json5format crate and its dependencies.

Refactored debug features from mod.rs to debug.rs

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