Standards and utilities for wrapping and unwrapping the aggregate constraint checker onion

[JoshOrndorff]

A typical Tuxedo runtime is aggregated from individual pieces. This aggregation may even be recursive if #116 works out).

It is common that you need to convert between an inner and outer constraint checker or vise versa. The aggregation macro automatically implements From<inner> for Outer but there is no convenient way to go from outer to inner. One idea is to have the macro implement methods like this.

// This is an expansion of what would be macro-generated code.
// I've elided the generic verifier for simplicity.
pub enum OuterConstraintChecker {
  Money(money::Money),
  Amoeba(amoeba::Amoeba),
}

// An assumption about both directions of conversion is that each
// constituent constraint checker is a unique type.

// These impls for wrapping layers onto the onion already exist.
impl From(money::Money) for OuterConstraintChecker { ... }
impl From(amoeba::Amoeba) for OuterConstraintChecker { ... }

// These ones could be added
impl From<OuterConstraintChecker> for money::Money { ... }
impl From<OuterConstraintChecker> for amoeba::Amoeba { ... }

A related issue is transforming a Transaction<OuterConstraintChecker> into a Transaction<InnerConstraintChecker> of vise versa. @muraca has given the idea of a transform method in 42f475c#diff-41e8e33684d0c2b86e827e306c10a1ce0787a681d8d9caf9191f178c09dfbe5eR23-R32 Previously I could only make that method work for wrapping the onion, but I think with the new From impls it would work both directions.