analyze: add scripts for computing pointwise metrics

[spernsteiner]

Adds scripts for computing "pointwise success rate" metrics. For each function, we run the static analysis and rewrite that function in isolation, producing a new .rs file where that function has been rewritten but all other code remains the same. Then we remove the unsafe qualifier from the target function and try to compile the code. The "pointwise success rate" is the number of functions on which this procedure succeeds.

The main entry point is c2rust-analyze/scripts/run_pointwise_metrics_lighttpd.sh (as the name suggests, this is designed to compute the success rate on lighttpd specifically). It uses a few helpers: pointwise_try_build.sh tries to remove unsafe and compile the rewritten code for a specific function, pointwise_try_build_unmodified.sh does the same but on the unmodified, non-rewritten code (used for computing a baseline success rate), and pointwise_metrics.py tallies up the results and prints overall counts.

Current output on lighttpd:

pointwise:     98/1008 functions passed
unmodified:   149/1008 functions passed
improved 20 functions
broke 71 functions

This PR depends on #1073, which implements the pointwise rewrite mode in c2rust-analyze.

[fw-immunant]

Deduplicate the nearly-identical scripts somehow, otherwise looks good.

[fw-immunant]

These are the only difference between these two scripts--it would be good to just make these conditional and avoid the duplication.

[spernsteiner]

I've updated this to deduplicate the similar scripts.