Merge type_map and restrictions into rmc-metadata.json

[tedinski]

We previously introduced type_map.json and restrictions.json as output files from rmc-rustc. And now for proof harnesses (and more) we have rmc-metadata.json.

We should merge the above two into rmc-metadata.json.

In the end, we'll just have:

1. symtab.json for cbmc's consumption.
2. rmc-metadata.json for consumption by our tools, which contains everything else we want to know.

As part of this, we should have a crate use to serialize/deserialize this format. Currently, this is mixed around in a few different places:

1. compiler/rustc_codegen_rmc/src/context/metadata.rs
2. library/rmc_restrictions/

We should create an rmc_metadata crate to house all these schemas.

[tedinski]

• Related Single use of type_map #1377

[carolynzech]

The only JSON file that Kani produces is kani-metadata.json, so this issue is complete.

[carolynzech]

Actually, we do generate type_map.json still, but I'm still inclined to leave this as closed just to keep kani-metadata.json smaller, especially since it's grown quite a bit in size since this issue was created.