Introduce ByteSymbol.

It's like `Symbol` but for byte strings. The interner is now used for
both `Symbol` and `ByteSymbol`. E.g. if you intern `"dog"` and `b"dog"`
you'll get a `Symbol` and a `ByteSymbol` with the same index and the
characters will only be stored once.

The motivation for this is to eliminate the `Arc`s in `ast::LitKind`, to
make `ast::LitKind` impl `Copy`, and to avoid the need to arena-allocate
`ast::LitKind` in HIR. The latter change reduces peak memory by a
non-trivial amount on literal-heavy benchmarks such as `deep-vector` and
`tuple-stress`.

`Encoder`, `Decoder`, `SpanEncoder`, and `SpanDecoder` all get some
changes so that they can handle normal strings and byte strings.

This change does slow down compilation of programs that use
`include_bytes!` on large files, because the contents of those files are
now interned (hashed). This makes `include_bytes!` more similar to
`include_str!`, though `include_bytes!` contents still aren't escaped,
and hashing is still much cheaper than escaping.

compiler/rustc_hir_pretty/src/lib.rs

@@ -1480,7 +1480,7 @@ impl<'a> State<'a> {
                 self.print_expr_addr_of(k, m, expr);
             }
             hir::ExprKind::Lit(lit) => {
-                self.print_literal(lit);
+                self.print_literal(&lit);
             }
             hir::ExprKind::Cast(expr, ty) => {
                 self.print_expr_cond_paren(expr, self.precedence(expr) < ExprPrecedence::Cast);