Simplify resolution of context and signature types

[dfreeman]

When a value of a given type is invoked in a template, glint needs a way to determine that value's signature—that is, what kind of template entity it behaves like, what kinds of inputs and outputs it expects, etc.

Similarly, when a template is associated with a given backing class, glint needs a way to determine what type of values {{this}}, {{@someArg}}, etc have based on that type.

Prior to this PR, that resolution was accomplished via indirect lookup against a registry in a similar manner to how fp-ts models typeclasses. This was necessary because the dependence on the template type to determine the overall type of a component required a high degree of flexibility in how we resolved signature and context types.

Now, with the changes in #15, knowing the instance type of the value in question is sufficient to determine everything we need to know about how it behaves in a template, so we can remove the HKT registry approach in favor of directly tagging the appropriate types on the instance using well-known symbols.