treat Unicode "replacement character" (i.e. decoding error) is malformed;

simplified/robustified treatment of malformed symbols, which are now fully internalized (total Symbol.explode etc.); allow malformed symbols inside quoted material, comments etc. -- for improved user experience with incremental re-parsing; refined treatment of malformed surrogates (Scala);

tuned;

back to quick_and_dirty, which is still practically important since the scheduler does not jump over subproofs;