introduce csize_t instead of fixing csize (#12497)

lib/system.nim

@@ -2271,7 +2271,9 @@ type # these work for most platforms:
     ## This is the same as the type ``short`` in *C*.
   cint* {.importc: "int", nodecl.} = int32
     ## This is the same as the type ``int`` in *C*.
-  csize* {.importc: "size_t", nodecl.} = int
+  csize* {.importc: "size_t", nodecl, deprecated: "use `csize_t` instead".} = int
+    ## This isn't the same as ``size_t`` in *C*. Don't use it.
+  csize_t* {.importc: "size_t", nodecl.} = uint
     ## This is the same as the type ``size_t`` in *C*.
   clonglong* {.importc: "long long", nodecl.} = int64
     ## This is the same as the type ``long long`` in *C*.
@@ -3638,7 +3640,7 @@ when not defined(JS): #and not defined(nimscript):
       when declared(memTrackerOp):
         memTrackerOp("copyMem", dest, size)
     proc moveMem(dest, source: pointer, size: Natural) =
-      c_memmove(dest, source, size)
+      c_memmove(dest, source, csize_t(size))
       when declared(memTrackerOp):
         memTrackerOp("moveMem", dest, size)
     proc equalMem(a, b: pointer, size: Natural): bool =
@@ -3656,7 +3658,7 @@ when not defined(JS): #and not defined(nimscript):
         else: result = 0
       else:
         let minlen = min(x.len, y.len)
-        result = int(nimCmpMem(x.cstring, y.cstring, minlen.csize))
+        result = int(nimCmpMem(x.cstring, y.cstring, cast[csize_t](minlen)))
         if result == 0:
           result = x.len - y.len