transmutability: Mark edges by ranges, not values

In the `Tree` and `Dfa` representations of a type's layout, store byte
ranges rather than needing to separately store each byte value. This
permits us to, for example, represent a `u8` using a single 0..=255 edge
in the DFA rather than using 256 separate edges.

This leads to drastic performance improvements. For example, on the
author's 2024 MacBook Pro, the time to convert the `Tree` representation
of a `u64` to its equivalent DFA representation drops from ~8.5ms to
~1us, a reduction of ~8,500x. See `bench_dfa_from_tree`.

Similarly, the time to execute a transmutability query from `u64` to
`u64` drops from ~35us to ~1.7us, a reduction of ~20x. See
`bench_transmute`.

compiler/rustc_transmute/src/lib.rs

@@ -1,8 +1,9 @@
 // tidy-alphabetical-start
+#![cfg_attr(test, feature(test))]
 #![feature(never_type)]
 // tidy-alphabetical-end
 
-pub(crate) use rustc_data_structures::fx::FxIndexMap as Map;
+pub(crate) use rustc_data_structures::fx::{FxIndexMap as Map, FxIndexSet as Set};
 
 pub mod layout;
 mod maybe_transmutable;