feat(kernel/environment): add forget method

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lua): add more level API unit tests

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lua): add more expr API unit tests

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lua): add more expr API unit tests