Use interned strings in mvc/model/query.c

Memory usage as reported by valgrind:
  * **Before:** total heap usage: 167,926 allocs, 97,608,376 bytes allocated
  * **After:** total heap usage: 167,095 allocs, 97,550,362 bytes allocated

Memory usage via `memory_get_usage()` (real / PHP):
  * **before:** (4194304) (4699096)
  * **after:** (3932160) (4639400)

Tested with:
  * `USE_ZEND_ALLOC=0 valgrind $(phpenv which php) unit-tests/manual-unit.php ModelsQueryParsingTest.php ModelsQueryParsingTest`
  * `$(phpenv which php) unit-tests/manual-unit.php ModelsQueryParsingTest.php ModelsQueryParsingTest`

The goal was to see how viable this optimization is — I did not try to replace all strings with interned ones.