move system.dollars in a separate file (#10829)

lib/system/strmantle.nim

@@ -293,7 +293,9 @@ proc nimCharToStr(x: char): string {.compilerRtl.} =
   result = newString(1)
   result[0] = x
 
-proc `$`(x: uint64): string =
+proc `$`*(x: uint64): string {.noSideEffect.} =
+  ## The stringify operator for an unsigned integer argument. Returns `x`
+  ## converted to a decimal string.
   if x == 0:
     result = "0"
   else: