Specify rust lints for compiler/ crates via Cargo.

By naming them in `[workspace.lints.rust]` in the top-level
`Cargo.toml`, and then making all `compiler/` crates inherit them with
`[lints] workspace = true`. (I omitted `rustc_codegen_{cranelift,gcc}`,
because they're a bit different.)

The advantages of this over the current approach:
- It uses a standard Cargo feature, rather than special handling in
  bootstrap. So, easier to understand, and less likely to get
  accidentally broken in the future.
- It works for proc macro crates.

It's a shame it doesn't work for rustc-specific lints, as the comments
explain.

compiler/rustc_serialize/Cargo.toml

@@ -16,3 +16,6 @@ thin-vec = "0.2.12"
 rustc_macros = { path = "../rustc_macros" }
 tempfile = "3.2"
 # tidy-alphabetical-end
+
+[lints]
+workspace = true