followup #18318: simplify dollarImpl and add a test (#18330)

lib/system/dollars.nim

@@ -7,15 +7,8 @@ proc `$`*(x: int): string {.magic: "IntToStr", noSideEffect.}
   ## spelling `toString`:idx:.
 
 template dollarImpl(x: uint | uint64, result: var string) =
-  type destTyp = typeof(x)
-  if x == 0:
-    return "0"
-  elif x == 1:
-    return "1"
-
   let length = digits10(x)
   setLen(result, length)
-
   numToString(result, x, length)
 
 when defined(js):

tests/stdlib/tdigitsutils.nim

@@ -0,0 +1,23 @@
+import std/private/digitsutils
+
+template main =
+  block: # digits10
+    doAssert digits10(0'u64) == 1
+    # checks correctness on all powers of 10 + [0,-1,1]
+    var x = 1'u64
+    var num = 1
+    while true:
+      # echo (x, num)
+      doAssert digits10(x) == num
+      doAssert digits10(x+1) == num
+      if x > 1:
+        doAssert digits10(x-1) == num - 1
+      num += 1
+      let xOld = x
+      x *= 10
+      if x < xOld:
+        # wrap-around
+        break
+
+static: main()
+main()