Safe Transmute: Check mutability before creating dst -> src obligation

- Only create dst -> src obligation if Dst is mutable
- Add some long comments to explain parts of the transmutability code that were
  unclear to me when reading
- Update/add tests

compiler/rustc_transmute/src/lib.rs

@@ -1,4 +1,4 @@
-#![feature(alloc_layout_extra, decl_macro, iterator_try_reduce, never_type, let_chains)]
+#![feature(alloc_layout_extra, decl_macro, iterator_try_reduce, never_type)]
 #![allow(dead_code, unused_variables)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]