move decode_helpers to std/private (#16209)

lib/std/private/decode_helpers.nim

@@ -0,0 +1,22 @@
+proc handleHexChar*(c: char, x: var int, f: var bool) {.inline.} =
+  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
+
+proc decodePercent*(s: openArray[char], i: var int): char =
+  ## Converts `%xx` hexadecimal to the character with ordinal number `xx`.
+  ##
+  ## If `xx` is not a valid hexadecimal value, it is left intact: only the
+  ## leading `%` is returned as-is, and `xx` characters will be processed in the
+  ## next step (e.g. in `uri.decodeUrl`) as regular characters.
+  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:
+      result = chr(x)
+      inc(i, 2)