typetraits: add toSigned, toUnsigned (#18445)

* typetraits: add toSigned, toUnsigned

* improve and add tests

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>
Co-authored-by: flywind <xzsflywind@gmail.com>

tests/stdlib/tbitops_utils.nim

@@ -0,0 +1,14 @@
+import std/private/bitops_utils
+
+template chk(a, b) =
+  let a2 = castToUnsigned(a)
+  doAssert a2 == b
+  doAssert type(a2) is type(b)
+  doAssert type(b) is type(a2)
+
+chk 1'i8, 1'u8
+chk -1'i8, 255'u8
+chk 1'u8, 1'u8
+chk 1'u, 1'u
+chk -1, cast[uint](-1)
+chk -1'i64, cast[uint64](-1)

tests/stdlib/ttypetraits.nim

@@ -2,10 +2,10 @@ discard """
   targets: "c cpp js"
 """
 
+# xxx merge with tests/metatype/ttypetraits.nim
+
 import std/typetraits
 
-
-
 macro testClosure(fn: typed, flag: static bool) =
   if flag:
     doAssert hasClosure(fn)