Adding machine info

[dkcumming]

The MachineInfo semantics productions are now connected to the JSON which now includes it with an update. This is ignored in the Pgm currently, but could be used to make isize and usize dynamic on pointer-width - although, I am not sure how to do that atm.

This is currently pointing to a different branch of stable mir json

[ehildenb]

We shouldn't merge with it pointing to a different branch of stable-mir-json, that should be merged first (which I think is not a breaking change here?)

Also, what are we using the machine info for? Specifically for constants? maybe we should process those in the Python first, so that we only build K terms that have the higher-level data present in them?

[dkcumming]

@ehildenb
I only had it pointing at a branch while I was figuring out how to get it working.
It is needed for knowing the bit width of a usize and isize as well. But I brought it into the K config for two reasons:

1. There were already the productions for it in target.md
2. I thought I would be able to add it to the K configuration and make usize and isize work with the appropriate bitwidth there instead of hard coding it to 64.

I couldn't see an easy way to achieve 2. (@gtrepta also helped and couldn't see a neat solution). But with all the parsing connected I figured I would open the PR. Do you think there is a way of getting around the usize and isize with python also?