Move MAX and MIN macros to macros.h

vim.h is too big, and these were missing parens around the arguments.
These may already be defined in <sys/param.h> or <windef.h>, so guard
them with an #ifndef.

extracted from #810

src/nvim/vim.h

@@ -8,9 +8,6 @@
 #ifndef NVIM_VIM_H
 #define NVIM_VIM_H
 
-#define MIN(X, Y) (X < Y ? X : Y)
-#define MAX(X, Y) (X > Y ? X : Y)
-
 #include "nvim/types.h"
 #include "nvim/pos.h"  // for linenr_T, MAXCOL, etc...