Remove long_u: hardcopy: Refactor long_u.

- <color_related_stuff>: long_u --> uint32_t

  Everywhere long_u was used to hold a color value.
  Color values are supposed to be 32 bits at most.
  Supported architectures have 32 bits ints, so we could have used plain
  ints. But this wouldn't be future-proof, and would be wasteful if a
  future architecture has ints bigger than 32 bits.
  So, uint32_t is perfect to achieve optimal packing no matter the
  architecture.

- bytes_to_print/bytes_printed: long_u --> size_t

  Seems like the correct thing, and gets rid of some casts.

src/nvim/hardcopy.h

@@ -1,12 +1,14 @@
 #ifndef NVIM_HARDCOPY_H
 #define NVIM_HARDCOPY_H
 
+#include <stdint.h>
+
 /*
  * Structure to hold printing color and font attributes.
  */
 typedef struct {
-  long_u fg_color;
-  long_u bg_color;
+  uint32_t fg_color;
+  uint32_t bg_color;
   int bold;
   int italic;
   int underline;