ISTF validator

[kof]

At some point, we should start with validator, which will validate the ISTF array for correctness.

Use cases

• It will be needed to control the output from CSS parser or from a objects->istf transformer.
• It will ease the life of those who try to implement their own *-to-istf compiler and want to be sure they are doing it right.
• One could use it to validate a package installed from npm with istf inside and make sure the produced istf is correct.

Tasks

• Make sure validator knows all constraints we put into ISTF Reducing ISTF spec to a CSS subset for consistency #44
• Make sure it outputs meaningful error messages