Fix transmutes to/from type with unspecified layout (Utf8Char)

This crate (among others) did'nt compile with an experimental [more strict
transmute check](rust-lang/rust#51294)

Hare's the error messages: (from http://cargobomb-reports.s3.amazonaws.com/pr-51294/5298b81bebd26bb9f87d5f99764aab01595b3f69-alt/reg/encode_unicode-0.3.1/log.txt)
Jul 05 13:02:02.884 INFO kablam! error[E0912]: transmutation to a type with an unspecified layout
Jul 05 13:02:02.884 INFO kablam!    --> src/utf8_char.rs:283:16
Jul 05 13:02:02.884 INFO kablam!     |
Jul 05 13:02:02.884 INFO kablam! 283 |             Ok(transmute(unused_zeroed))
Jul 05 13:02:02.884 INFO kablam!     |                ^^^^^^^^^
Jul 05 13:02:02.884 INFO kablam!     |
Jul 05 13:02:02.884 INFO kablam!     = note: utf8_char::Utf8Char has an unspecified layout
Jul 05 13:02:02.884 INFO kablam!
Jul 05 13:02:02.884 INFO kablam! error[E0912]: transmutation from a type with an unspecified layout
Jul 05 13:02:02.884 INFO kablam!   --> src/utf8_iterator.rs:25:41
Jul 05 13:02:02.884 INFO kablam!    |
Jul 05 13:02:02.884 INFO kablam! 25 |         let used = u32::from_le(unsafe{ mem::transmute(uc) });
Jul 05 13:02:02.884 INFO kablam!    |                                         ^^^^^^^^^^^^^^
Jul 05 13:02:02.884 INFO kablam!    |
Jul 05 13:02:02.884 INFO kablam!    = note: utf8_char::Utf8Char has an unspecified layout
Jul 05 13:02:02.884 INFO kablam!
Jul 05 13:02:02.893 INFO kablam! error: aborting due to 2 previous errors

Author: tormol

src/utf8_char.rs

@@ -282,7 +282,7 @@ impl Utf8Char {
             let extra = utf8[0].extra_utf8_bytes_unchecked() as u32;
             let mask = u32::from_le(0xff_ff_ff_ff >> 8*(3-extra));
             let unused_zeroed = mask  &  transmute::<_,u32>(utf8);
-            Ok(transmute(unused_zeroed))
+            Ok(Utf8Char{ bytes: transmute(unused_zeroed) })
         }
     }
     /// Unused bytes must be zero

src/utf8_iterator.rs

@@ -22,7 +22,7 @@ pub struct Utf8Iterator (u32);
 
 impl From<Utf8Char> for Utf8Iterator {
     fn from(uc: Utf8Char) -> Self {
-        let used = u32::from_le(unsafe{ mem::transmute(uc) });
+        let used = u32::from_le(unsafe{ mem::transmute(uc.to_array().0) });
         // uses u64 because shifting an u32 by 32 bits is a no-op.
         let unused_set = (u64::MAX  <<  uc.len() as u64*8) as u32;
         Utf8Iterator(used | unused_set)