Add TRACING_ENABLED to Machine and add enter_trace_span!()

[Stypox]

This PR adds the necessary infrastructure to make it possible to do tracing calls from within rustc_const_eval when running Miri, while making sure they don't impact the performance of normal compiler execution. This is done by adding a const boolean to Machine, false by default, but that will be set to true in Miri only. The tracing macro enter_trace_span!() checks if it is true before doing anything, and since the value of a const is known at compile time, if it it false it the whole tracing call should be optimized out.

I will soon open further PRs to add tracing macro calls similar to this one, so that afterwards it will be possible to learn more about Miri's time spent in the various interpretation steps:

let _guard = enter_trace_span!(M, "eval_statement", "{:?}", stmt);

r? @RalfJung

[rustbot]

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri