Auto merge of rust-lang#98649 - RalfJung:guardians-of-mir, r=oli-obk

move MIR syntax into a dedicated file and ping some people whenever it changes

Adding or changing MIR operations/statements/whatever should be under significant scrutiny wrt their wider impact, specified semantics, and so on. So let's start by putting all that into a dedicated file and pinging some people whenever that file changes.

This PR only moves definitions around, and then fiddles with imports until it all works again.

Author: bors