transmutability: remove NFA intermediate representation

Prior to this commit, the transmutability analysis used an intermediate
NFA representation of type layout. We then determinized this
representation into a DFA, upon which we ran the core transmutability
analysis. Unfortunately, determinizing NFAs is expensive. In this
commit, we avoid NFAs entirely by observing that Rust `union`s are the
only source of nondeterminism and that it is comparatively cheap to
compute the DFA union of DFAs.

We also implement Graphviz DOT debug formatting of DFAs.

Fixes rust-lang/project-safe-transmute#23
Fixes rust-lang/project-safe-transmute#24

compiler/rustc_transmute/src/layout/mod.rs

@@ -4,9 +4,6 @@ use std::hash::Hash;
 pub(crate) mod tree;
 pub(crate) use tree::Tree;
 
-pub(crate) mod nfa;
-pub(crate) use nfa::Nfa;
-
 pub(crate) mod dfa;
 pub(crate) use dfa::Dfa;
 
@@ -29,6 +26,13 @@ impl fmt::Debug for Byte {
     }
 }
 
+#[cfg(test)]
+impl From<u8> for Byte {
+    fn from(src: u8) -> Self {
+        Self::Init(src)
+    }
+}
+
 pub(crate) trait Def: Debug + Hash + Eq + PartialEq + Copy + Clone {
     fn has_safety_invariants(&self) -> bool;
 }