Simplify Ctxt lookups in ASTUtils

This patch makes context lookups more straightforward by removing the `InitialValue` typeclass — it turns out that the same type might have different appropriate initial values in different contexts and the default value should be given in the corresponding lens instead. Also, this approach does not cause nonsense like `initialValue = error "blah blah does not exist"` (shows up in my unpublished `open Record` code) and it lets the programmer have finer control over missing values.