memory: Add free wrapper and refactor project to use it

We already use wrappers for allocation, the new `xfree` function is the
equivalent for deallocation and provides a way to fully replace the malloc
implementation used by Neovim.

src/nvim/misc2.c

@@ -329,10 +329,10 @@ int call_shell(char_u *cmd, ShellOpts opts, char_u *extra_shell_arg)
           : STRCMP(p_sxq, "\"(") == 0 ? (char_u *)")\""
           : p_sxq);
       retval = os_call_shell(ncmd, opts, extra_shell_arg);
-      free(ncmd);
+      xfree(ncmd);
 
       if (ecmd != cmd)
-        free(ecmd);
+        xfree(ecmd);
     }
   }
 
@@ -387,7 +387,7 @@ int vim_chdir(char_u *new_dir)
   if (dir_name == NULL)
     return -1;
   r = os_chdir((char *)dir_name);
-  free(dir_name);
+  xfree(dir_name);
   return r;
 }
 
@@ -453,7 +453,7 @@ char *read_string(FILE *fd, size_t cnt)
   for (size_t i = 0; i < cnt; i++) {
     int c = getc(fd);
     if (c == EOF) {
-      free(str);
+      xfree(str);
       return NULL;
     }
     str[i] = (uint8_t)c;