string.h: don't pollute the namespace with locale.h

Previously, we included all of `<locale.h>` in order to get `locale_t`.
This commit separates it into a `<bits/locale-t.h>` header (generated by
cbindgen) to better deal with this.

src/header/string/mod.rs

@@ -20,7 +20,7 @@ use crate::{
     raw_cell::RawCell,
 };
 
-use super::locale::{THREAD_LOCALE, locale_t};
+use super::{bits_locale_t::locale_t, locale::THREAD_LOCALE};
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/memccpy.html>.
 ///