Cleanup: hlint suggestions and some formatting

Made a number of changes suggested by hlint, deleted some old
commented-out code and reformatted some long lines.

No change in functionality.
Remove debug trace
Report correct input file name with parse errors.

Parse errors now contain the name of the file containing the
error, rather than "TODO".
-o option now works

If the "-o" option is specified, funk now writes the output
(LLVM IR, currently) to the given file. Previously, the IR was
always printed to stdout, even if an output file was given.
Minor updates to the description of Funk in README