--version should also print the mir-json version

[sauclovian-g]

Currently if you ask the mir-json tools for their version they print the rustc version, which is important, but realistically they should also print the mir-json version. Maybe also the current JSON schema version too, now that we have one.

In order to do this we need a version, but there is actually one in the cargo metadata (maybe we should bump it for the recent JSON changes?) so this shouldn't be that difficult to arrange.

[RyanGlScott]

I agree that we should do this. I'm unclear if the rustc API offers a clean way to override the behavior of the --version flag, but if nothing else, we could reimplement --version ourselves. (The rustc implementation can be found here.)

[spernsteiner]

I'm unclear if the rustc API offers a clean way to override the behavior of the --version flag

Doesn't look like it - the version string is baked in when librustc_interface is compiled: https://github.com/rust-lang/rust/blob/9110d925d0b3e4842376e830f4404a28e1aa2658/compiler/rustc_interface/src/util.rs#L654-L657