fix interpreter tracing output https://github.com/rust-lang/rust/pull/144708 accidentally changed the output of `MIRI_LOG=info <miri run>` in two ways: - by adding `stmt=`/`terminator=` prefixes - by changing the statement printing to be a verbose debug version instead of MIR pretty-printing This fixes both of these: - use explicit format strings to avoid the prefixes - fix inconsistency in Debug impls for MIR types: now both TerminatorKind and StatementKind are pretty-printed, and Terminator and Statement just forward to the *Kind output