Bind PartitionSpec to its schema #2530

When writing logic for rewriting manifest files, it was found to be easy to hand partition specs around with the wrong schema reference: #2367 (comment)

This issue proposes to bind a PartitionSpec to a schema, such as by holding a reference to the Schema. Or, a better approach if there is one.

Ultimately, rough idea:

• Add schema to PartitionSpec
• Update SnapshotSummaryCollector's APIs: schema should be inferred from partition spec, not passed in