Prioritize embedded string in the following functions:

- `str_new_static`
- `str_new`
- `mrb_str_new_capa`
- `mrb_str_pool`

The reasons are as follows:

- Consistency with `mrb_str_byte_subseq` and `str_replace`.
- Memory locality increases and may be slightly faster.
- No conversion cost to embedded string when modifying the string.