Optimized IR generation for UTF-8 and UTF-16 encoding

- Both can now be inlined and constant folded away
- Both can no longer cause failure
- Both now return an `Option` instead

Removed debug `assert!()`s over the valid ranges of a `char`
- It affected optimizations due to unwinding
- Char handling is now sound enought that they became uneccessary

src/libcoretest/char.rs

@@ -173,7 +173,7 @@ fn test_escape_unicode() {
 fn test_encode_utf8() {
     fn check(input: char, expect: &[u8]) {
         let mut buf = [0u8, ..4];
-        let n = input.encode_utf8(buf /* as mut slice! */);
+        let n = input.encode_utf8(buf.as_mut_slice()).unwrap_or(0);
         assert_eq!(buf.slice_to(n), expect);
     }
 
@@ -187,7 +187,7 @@ fn test_encode_utf8() {
 fn test_encode_utf16() {
     fn check(input: char, expect: &[u16]) {
         let mut buf = [0u16, ..2];
-        let n = input.encode_utf16(buf /* as mut slice! */);
+        let n = input.encode_utf16(buf.as_mut_slice()).unwrap_or(0);
         assert_eq!(buf.slice_to(n), expect);
     }