file.c: Optimize `rb_file_dirname_n` fixed costs

- `str_null_check` was performed twice, once by `FilePathStringValue`
  and a second time by `StringValueCStr`.
- `StringValueCStr` was checking for the terminator presence, but we
  don't care about that.
- `FilePathStringValue` calls `rb_str_new_frozen` to ensure `fname`
  isn't mutated, but that's costly for such a check. Instead we
  can do it in debug mode only.
- `rb_enc_get` is slow because it accepts arbitrary objects, even immediates,
  so it has to do numerous type checks. Add a much faster `rb_str_enc_get`
  when we know we're dealing with a string.
- `rb_enc_copy` is slow for the same reasons, since we already have the
  encoding, we can use `rb_enc_str_new` instead.

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