Convert the CDK assertion library to jsii so it will be available in all languages

[eladb]

The @aws-cdk/assert library is not a jsii library at the moment, as it has global functions that cannot be expressed in jsii. This means that it is impossible to package it to non-typescript languages. We should refactor it so that it will use static methods instead of global functions (i.e. Assert.expect(stack) instead of expect(stack)) and turn it into a jsii module so it will be available in all languages.

[RomainMuller]

The actual solution might instead take the form of making an assertions helper library, that facilitates using any languages' assertion framework with the CDK.

[lalitkale]

Any idea when this helper library will be available for .net or python? Is it considered as part of next release?

[eladb]

Duplicate #4