use hexchar in stdlib (#16290)

lib/std/private/decode_helpers.nim

@@ -1,9 +1,25 @@
-proc handleHexChar*(c: char, x: var int, f: var bool) {.inline.} =
+proc handleHexChar*(c: char, x: var int): bool {.inline.} =
+  ## Converts `%xx` hexadecimal to the ordinal number and adds the result to `x`.
+  ## Returns `true` if `c` is hexadecimal.
+  ##
+  ## When `c` is hexadecimal, the proc is equal to `x = x shl 4 + hex2Int(c)`.
+  runnableExamples:
+    var x = 0
+    assert handleHexChar('a', x)
+    assert x == 10
+
+    assert handleHexChar('B', x)
+    assert x == 171 # 10 shl 4 + 11
+
+    assert not handleHexChar('?', x)
+    assert x == 171 # unchanged
+  result = true
   case c
   of '0'..'9': x = (x shl 4) or (ord(c) - ord('0'))
   of 'a'..'f': x = (x shl 4) or (ord(c) - ord('a') + 10)
   of 'A'..'F': x = (x shl 4) or (ord(c) - ord('A') + 10)
-  else: f = true
+  else:
+    result = false
 
 proc decodePercent*(s: openArray[char], i: var int): char =
   ## Converts `%xx` hexadecimal to the character with ordinal number `xx`.
@@ -14,9 +30,6 @@ proc decodePercent*(s: openArray[char], i: var int): char =
   result = '%'
   if i+2 < s.len:
     var x = 0
-    var failed = false
-    handleHexChar(s[i+1], x, failed)
-    handleHexChar(s[i+2], x, failed)
-    if not failed:
+    if handleHexChar(s[i+1], x) and handleHexChar(s[i+2], x):
       result = chr(x)
       inc(i, 2)