Refactor Schema initialization in the codebase

[jaychia]

Schema() is currently initialized with Fields in its __init__. However, in schema2.py this will no longer be allowed since the new schemas are Rust datastructures.

This refactor changes all locations where we do this intialization to a call to Schema._from_field_name_and_types instead, which takes in a simple Python list of tuples, so that integration will be easier.