librustc: Forbid .. in range patterns.

This breaks code that looks like:

    match foo {
        1..3 => { ... }
    }

Instead, write:

    match foo {
        1...3 => { ... }
    }

Closes #17295.

[breaking-change]

src/libstd/num/strconv.rs

@@ -199,8 +199,8 @@ pub fn int_to_str_bytes_common<T: Int>(num: T, radix: uint, sign: SignFormat, f:
             current_digit_signed
         };
         buf[cur] = match current_digit.to_u8().unwrap() {
-            i @ 0..9 => b'0' + i,
-            i        => b'a' + (i - 10),
+            i @ 0...9 => b'0' + i,
+            i         => b'a' + (i - 10),
         };
         cur += 1;