chore: various renames and fixes

[Jmgr]

Changes

• removes a few class attributes definition (they should be instance attributes instead)
• renames inner to child (producer was also considered, but child is more idiomatic in the context of an AST)
• renames inner_type to contained_type in compound types (we are now storing more than one type in compound types) and removed inner_type from operations
• renames to_type to to_mir
• adds is_scalar and is_literal (will be used in a follow up PR)

Note that this doesn't break the MIR. We will do the necessary changes in a follow up PR, as well as in another PR in the main nillion repo so that it is able to interpret the changes.

You can review each commit separately instead of all changes at once if you want, should be easier to read.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Backwards compatibility analysis completed (if applicable). "Will this change require recompilation and upload of user programs?"

[Jmgr]

Sorry I had to make another change to this PR. As we discussed in Slack making breaking changes to the MIR now is not ideal, and we will also need to rename inner_type in a lot of places, so for now we are going to keep internal changes only.