autocorres: trivial: Tweak incorrect comment in "simpl_conv.ML".
autocorres: trivial: Remove unused function in "simpl_conv.ML".
lib: set: Add "filter" function for sets.

Can be more efficient than "Symset.dest x |> filter |> Symset.make"
because we know that the ordering is unmodified by filter.
autocorres: Translate missing functions into constant "FUNCTION_BODY_NOT_IN_INPUT_C_FILE".

Functions that are declared in the C file, called by other C functions,
but are never actually _defined_ are translated simply into a "fail"
monadic statement. This sometimes causes confusion to new users.

We update AutoCorres to instead emit a new constant:

  FUNCTION_BODY_NOT_IN_INPUT_C_FILE

defined simply as "fail" for such functions.