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/lib.rs

@@ -2,7 +2,7 @@
 #![feature(never_type)]
 // tidy-alphabetical-end
 
-pub(crate) use rustc_data_structures::fx::{FxIndexMap as Map, FxIndexSet as Set};
+pub(crate) use rustc_data_structures::fx::FxIndexMap as Map;
 
 pub mod layout;
 mod maybe_transmutable;