Experiment with new `FieldProxy` concept

Prototype this idea and see if it is valuable:

`FieldProxy` is not a type but an associated type of the `FieldType` trait.
=> each `FieldType` could use its own `FieldProxy` and directly implement methods as desired.
This removes the weird dance between `FieldProxy` and `FieldEq` (`FieldOrd`, etc).
It also simplifies the `Deref` implementation for "container" fields.