(feat) Type Inference Bounds (#73)

### quick overview

This PR is the next step for type inference in our DSL, and follows #72
.

To solve the constraints, we need two core functions:
`greatest_lower_bound` and `least_upper_bound`.
- The least upper bound is the most specific type that is a supertype of
both input types.
- The greatest lower bound is the most general type that is a subtype of
both input types.
See tests for examples.

Furthermore, some code in `types.rs` was broken, but is now fixed.
- Map key was not contravariant, but covariant.
- `T` was not a subtype of `Optional[T]`.

### next steps

Writing the solver is the next and final step for practical type
inference. We would still need to handle `Generics` properly, and
propagate `Costed`/`Stored`.

Author: AlSchlo

optd-dsl/Cargo.toml

@@ -23,4 +23,4 @@ futures = "0.3.31"
 ordered-float = "5.0.0"
 
 [dev-dependencies]
-tokio = { version = "1.44.2", features = ["macros"] }
+tokio = { version = "1.44.2", features = ["macros", "rt-multi-thread", "test-util"] }