Rework type conversions in macro language

[EvanKirshenbaum]

While trying to implement prompt (#219), and reworking the injection logic, it became clear that the type conversion scheme is in drastic need of an overhaul.

In particular, it would be nice if

1. I could just find the widest functional type for a (potentially non-functional) type such as DIR or ON, so that I didn't need to have special cases everywhere.
2. CallableValue should be able to provide a wrapper value to take the types it is provided and do conversions on input and output automatically.
3. CallableValue should do proper narrowing (covariant param types, contravariant return types)
4. Representation types should be stored with the types.
5. Conversion functions should be transitive when registered and implicitly inferred based on representation.
6. Conversion functions should be cached with the source type to prevent all the expensive logic.

   Migrated from internal repository. Originally created by @EvanKirshenbaum on Jan 24, 2023 at 12:02 PM PST. Closed on Jan 26, 2023 at 5:38 PM PST.

[EvanKirshenbaum]

This issue was referenced by the following commits before migration:

• 8f5bd5d88dd02a575de12f4c9e538cf61addea0e by @EvanKirshenbaum on Jan 24, 2023 at 3:37 PM PST
• 2c5a6dc4b3bb436e1b7b08617316ba351658acf8 by @EvanKirshenbaum on Jan 26, 2023 at 5:28 PM PST
• e365df56a69708cce3dadd8b7b5be1b44de8cf2d by @EvanKirshenbaum on Jan 28, 2023 at 4:58 PM PST

[EvanKirshenbaum]

Everything's all nice and refactored, though not tremendously well tested.

• Types now know their CallableType super, which means that I could get rid of all the special-casing for DIR, ON, and OFF in visitInjection().
• CallableType (actually Signature) now creates a wrapper CallableValue when it needs to convert.
• Conversion functions are elided if it can be reasoned that passing the value should work.
• < now reasons based on whether or not it can find a conversion.
• Conversion is fully transitive. If it knows how to get from A to B and from B to C, it can get from A to C.
• For going from a type to its direct super, if there is no explicit conversion, it checks whether the representations are compatible.
• Conversion functions are cached.
• To make conditionals without else clauses work, conditionals now use upper_bound() to figure out their return type, which is probably the right thing to do.
• upper_bound() caches its results. Also, efficient short-circuit cases for when all of the arguments are the same (modulo "maybe").

Migrated from internal repository. Originally created by @EvanKirshenbaum on Jan 26, 2023 at 4:59 PM PST.