core / init.meta.local_context
A local context is a list of local constant declarations.
Each metavariable in a metavariable context holds a local_context
to declare which locals the metavariable is allowed to depend on.
The empty local context.
Add a new local constant to the lc. The new local has an unused unique_name.
Fails when the type depends on local constants that are not present in the context.