feat(frontends/lean/elaborator): use local declarations as class instances

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/builtin_exprs): add 'including' expression

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/elaborator): support for local instances

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>