Use ordsets in apply_mappings() fold

The fold function in apply_mappings() would simply cons mappings
to lists for either dropping or keeping a mapping, and then
subtracting the two. The effect was, in a sense, a majority vote:
if more mappings than not indicated dropping, the translation
would be dropped, counter-intuitively.

Solution: make the accumulators ordsets instead. Two test cases
were added: one with 1 'good mapping' and 1 'bad', and the other
with 1 'good' and 2 'bad'. The second test failed before the fix:

```
uwair:cuttlefish uwiger$ rebar eunit skip_deps=true suites=cuttlefish_generator
…
  cuttlefish_generator: apply_mappings_translations_dropped_correctly_mixed2_test...*failed*
in function cuttlefish_generator:'-apply_mappings_translations_dropped_correctly_mixed2_test/0-fun-2-'/2 (src/cuttlefish_generator.erl, line 948)
in call from cuttlefish_generator:apply_mappings_translations_dropped_correctly_mixed2_test/0 (src/cuttlefish_generator.erl, line 948)
**error:{assertEqual_failed,[{module,cuttlefish_generator},
                     {line,948},
                     {expression,"TranslationsToDrop"},
                     {expected,[]},
                     {value,["mapping.name"]}]}
=======================================================
  Failed: 1.  Skipped: 0.  Passed: 23.
```
After the fix:

```
uwair:cuttlefish uwiger$ rebar eunit skip_deps=true suites=cuttlefish_generator
…
=======================================================
  All 24 tests passed.
```