Fixes panics after remove raised in
#9

Added new tests, checked they failed

Fixed by making lookup functions handle removed buckets. Fix was needed
in lookp and lookup_mut.  Was able to generalize the old lookup_or_free
to cover all lookup cases by parameterizing the lock->guard function.

Deleted dead code no longer needed due to above

Tested using cargo test:
test result: ok. 34 passed; 0 failed; 0 ignored; 0 measured; 0 filtered
out