Remove project int types: Case short_u: Replace with uint16_t.

- Replace short_u with uint16_t (same size, should give no problems).
- When possible, remove explicit downcasts so that they are found when
  flag -Wconversion enabled.
- Remove short_u typedef.

Requested in #459.

src/syntax_defs.h

@@ -69,8 +69,8 @@ typedef struct attr_entry {
     } term;
     struct {
       /* These colors need to be > 8 bits to hold 256. */
-      short_u fg_color;                 /* foreground color number */
-      short_u bg_color;                 /* background color number */
+      uint16_t fg_color;                /* foreground color number */
+      uint16_t bg_color;                /* background color number */
     } cterm;
   } ae_u;
 } attrentry_T;