Treat unicode hex digits case-insensitively

In Rust >= 1.24.0 we could have used `char::is_ascii_hexdigit`, but to keep
compatiblity with older versions, `char::is_digit(16)` is used.

Fixes #240.

src/tokens.rs

@@ -2,6 +2,7 @@ use std::borrow::Cow;
 use std::char;
 use std::str;
 use std::string;
+use std::string::String as StdString;
 
 use self::Token::*;
 
@@ -369,19 +370,15 @@ impl<'a> Tokenizer<'a> {
     }
 
     fn hex(&mut self, start: usize, i: usize, len: usize) -> Result<char, Error> {
-        let mut val = 0;
+        let mut buf = StdString::with_capacity(len);
         for _ in 0..len {
             match self.one() {
-                Some((_, ch)) if '0' <= ch && ch <= '9' => {
-                    val = val * 16 + (ch as u32 - '0' as u32);
-                }
-                Some((_, ch)) if 'A' <= ch && ch <= 'F' => {
-                    val = val * 16 + (ch as u32 - 'A' as u32) + 10;
-                }
+                Some((_, ch)) if ch as u32 <= 0x7F && ch.is_digit(16) => buf.push(ch),
                 Some((i, ch)) => return Err(Error::InvalidHexEscape(i, ch)),
                 None => return Err(Error::UnterminatedString(start)),
             }
         }
+        let val = u32::from_str_radix(&buf, 16).unwrap();
         match char::from_u32(val) {
             Some(ch) => Ok(ch),
             None => Err(Error::InvalidEscapeValue(i, val)),

test-suite/tests/valid/unicode-escape.json

@@ -1,5 +1,8 @@
 {
     "answer1": {"type": "string", "value": "\u000B"},
     "answer4": {"type": "string", "value": "\u03B4α"},
-    "answer8": {"type": "string", "value": "\u03B4β"}
+    "answer8": {"type": "string", "value": "\u03B4β"},
+    "answer9": {"type": "string", "value": "\uc0de"},
+    "answer10": {"type": "string", "value": "\u03B4α"},
+    "answer11": {"type": "string", "value": "\uABC1"}
 }

test-suite/tests/valid/unicode-escape.toml

@@ -1,3 +1,6 @@
 answer1 = "\u000B"
 answer4 = "\u03B4α"
 answer8 = "\U000003B4β"
+answer9 = "\uc0de"
+answer10 = "\u03b4α"
+answer11 = "\U0000abc1"