Add units to schema

[porcuquine]

We currently track units informally as part of parameter description in schema. Add new metadata for units/types (needs design) to formalize this.

• v1: Initially this only needs to be usable for formatting
  • [ ] display uses human-readable times, storage sizes, dollar amounts, etc.
• v2: later, this can be used to
  • [ ] eliminate some intermediate values
  • [ ] provide automated conversions
  • [ ] provide some typechecking. (e.g. distance / time = speed, etc.)