feat: [WIP] Persistent env extension custom types

[digama0]

The PersistentEnvExtension type required that the data stored in an .olean file for the extension was of type Array α for some α, even though it did not really take advantage of this structure at all (except for requiring that it has a default value, for slightly dubious reasons). This PR generalizes it to allow any type, in case the extension wants to e.g. store a per-file HashMap or RBMap directly in the olean instead of an array.

Breaking changes (metaprogramming only):

• All uses of PersistentEnvExtension α β σ are now PersistentEnvExtension (Array α) β σ. Other extension variants are not affected.