Support associated const formulas for blanket impls

Blanket impls currently store concrete `Vec` associated const values, which is insufficient when a const depends on generic parameters, such as `Value::SIZE` or `Value::ALIGN` for `Value for [T] where T: Value`.

Currently the compiler has a work-around for `Value::SIZE` and `Value::ALIGN`. But obviously that doesn't scale.

We should add an associated-const template/formula representation for blanket impls, substitute concrete types during materialization, and evaluate the formulas into concrete dictionary values. This should replace the current `Value`-specific layout synthesis special case.

No files, tests, or entry points are named. Start by tracing the blanket-impl representation, materialization, const evaluation, and the current Value-specific layout synthesis workaround. Done means generic associated-const formulas are substituted and evaluated for concrete blanket impls, replacing that special case.

Written by the indexing model from the issue text.