analyze: static/dynamic comparison mode for NON_NULL (#1151)

This adds a mode for comparing static analysis results to the dynamic
results from the PDG. Comparison mode is enabled by setting the env var
`C2RUST_ANALYZE_COMPARE_PDG=1`. When this mode is enabled,
c2rust-analyze skips normal rewriting and instead prints a copy of the
input code with annotation comments indicating places where static and
dynamic NON_NULL results disagree. Specifically, it annotates cases
where either the static analysis computed NON_NULL but the PDG includes
at least one observation where the pointer is null, or the dynamic
analysis marked the pointer nullable but the PDG contains observations
that are all non-null.

In support of the main change, this branch also includes a refactor of
the PDG processing code.