[type1, cid, type42] Streamline dictionary parsing.

When matching the keywords, we avoid calculating their lengths by
checking the stored values. This itself is a sufficient pre-check
before diving into `memcmp`. Therefore, we remove explicit check of
the first characters.

* include/freetype/internal/psaux.h (T1_FieldRec): Store length.
* src/cid/cidload.c (cid_parse_dict): Use `memcmp` and stored length.
* src/type1/t1load.c (parse_dict): Ditto.
* src/type42/t42parse.c (t42_parse_dict): Ditto.