Make `optimization-coach-profile' use errortrace-based profiling by default.
Clean up profile loading API.
Allow use of profiling info at the command-line.
Convert errortrace-based expression profiles to function profiles.

Inlining analysis needs function-level granularity (graphs with
functions as nodes and calls as edges), and pruning also benefits from
coarser granularity.

Finer-grained expression profiles can certainly be useful (e.g. to
profile different paths inside a function), but using them is future
work.
Report safe fx operations in hot code.
Remove dependency on images docs.
Now that OC depends on Racket 6.0, no need for our own group-by.
Convert README to markdown.