Optimization in TailCalls

Only store the position and reason for a failure to tailcall
transform a method if we ever need to report it, ie, if the
method was annotated with @tailrec.

Saves object hashing and map updates, profiling suggests
that this reduces the tailcalls phase from about 2% of compilation
time to about 1%.

Also, clear the maps eagerly after each compilation unit,
rather than letting them accumulate entries for the entire
run. Working with smaller maps can't hurt.