make cuchar alias uint8 instead of char (#17738)

changelog.md

@@ -4,6 +4,8 @@
 
 ## Changes affecting backward compatibility
 
+-  `cuchar` now aliases `uint8` instead of `char`
+
 - `repr` now doesn't insert trailing newline; previous behavior was very inconsistent,
   see #16034. Use `-d:nimLegacyReprWithNewline` for previous behavior.
 

lib/system.nim

@@ -1452,7 +1452,7 @@ type # these work for most platforms:
     ## This is the same as the type `long double` in *C*.
     ## This C type is not supported by Nim's code generator.
 
-  cuchar* {.importc: "unsigned char", nodecl.} = char
+  cuchar* {.importc: "unsigned char", nodecl.} = uint8
     ## This is the same as the type `unsigned char` in *C*.
   cushort* {.importc: "unsigned short", nodecl.} = uint16
     ## This is the same as the type `unsigned short` in *C*.