*: Move some dictionary functions to typval.h and use char*

Also fixes buffer reusage in setmatches() and complete().

src/nvim/charset.h

@@ -5,6 +5,16 @@
 #include "nvim/pos.h"
 #include "nvim/buffer_defs.h"
 
+/// Return the folded-case equivalent of the given character
+///
+/// @param[in]  c  Character to transform.
+///
+/// @return Folded variant.
+#define CH_FOLD(c) \
+    utf_fold((sizeof(c) == sizeof(char)) \
+             ?((int)(uint8_t)(c)) \
+             :((int)(c)))
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "charset.h.generated.h"
 #endif