Enforce that there is exactly one field name per field.

Currently, we silently create an invalid struct if the number of
field names does not match the number of fields. Unsurprisingly,
these objects then fail in confusing ways.

Avoid this by checking at struct definition time that the number
of field names matches the number of actual fields.

Test: new unit test passes
Test: all_tests.sh passes on android-4.9
Change-Id: If0f597f89c7dd92c785d1d19d3fcb5daa93c8d65