Support backward compatibility checking

[thisfred]

As a developer using val I want to be able to automatically verify if modifications to one of my schema's will be backwards compatible with a previous version.

so something like:

schema_v1 = Schema({
    "username": str,
    "password": str})

schema_v2 = Schema({
    "username": str,
    "one_time_code": str})

print(is_backward_compatible(schema_v2, schema_v1)
False

schema_v2 = Schema({
    "username": str,
    Optional(password): str,
    Optional(one_time_code): str})

print(is_backward_compatible(schema_v2, schema_v1)
True