refactor: the long goodbye

long is 32 bits on windows, while it is 64 bits on other architectures.
This makes the type suboptimal for a codebase meant to be
cross-platform. Replace it with more appropriate integer types.

src/nvim/mbyte.c

@@ -78,8 +78,8 @@ typedef struct {
 } convertStruct;
 
 struct interval {
-  long first;
-  long last;
+  int first;
+  int last;
 };
 
 // uncrustify:off