Move wcwidth.c and utf8.{h,c} to core.

This move makes sense since these files contain rather fundamental functions
(fundamental here means that we shouldn't have had to implement them)
which are required by other functions located in core/special-vars.c.

src/core/Makefile.am

@@ -44,6 +44,8 @@ libcore_a_SOURCES = \
 	settings.c \
 	signals.c \
 	special-vars.c \
+	utf8.c \
+	wcwidth.c \
 	write-buffer.c
 
 structure_headers = \