feat: Add storage slots to globals

[jfecher]

Related issue(s)

Resolves #1018

Description

Summary of changes

This PR assigns a storage slot to each global, starting from 1, and resetting back each time a new contract is identified.

Note that this PR does not add a way to actually access this value within noir code yet. I expect this to be more difficult since we will need to track whether e.g. a parameter of a function will have its storage slot taken, requiring us to verify the function is either called with a global as its argument or with another parameter.

Dependency additions / changes

Test additions / changes

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

Documentation needs

• This PR requires documentation updates when merged.

Additional context