Compile without serde

[RalfJung]

Can we get rid of the dependency on serde? That would enable using a stage 1 compiler to build miri, which would save me a lot of time.

[RalfJung]

According to @eddyb, serde is pulled in via cargo_metadata for "cargo miri". So I suggest we either make cargo miri a feature or a subcrate. Also see #166.

[oli-obk]

if we can make it a feature I'd prefer that.

[RalfJung]

Unfortunately using the stage 1 compiler still does not work. It doesn't find librustc.

[RalfJung]

Also the test suite needs serde to parse rustc JSON output.

So, this is unlikely to ever work without proc macros.