refactor(kernel/context): hide that the context is implemented using lists

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/elaborator): bug at method process_metavar_inst, add new test that exposed the bug

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/elaborator): bug at method process_metavar_inst, add new test that exposed the bug