Tokens (such as resource attributes) can now be implicitly converted
into strings. Parameters and attributes can now be combined into larger
strings using the native string facilities of the host language.

The process will be reversed by the resolve() function during synthesis,
which will split the string up again into string literals and
CloudFormation intrinsics.

The same transformation is applied during JSON.stringify(), so that
Tokens in a complex JSON structure are preserved.

Fixes #24 and #168.

MAJOR CHANGES

- Reintroducing tokenAwareJsonify; it needs to be there because
  otherwise we cannot have enough knowledge to do escaping properly;
  see documentation in the code.
- Introducing markAsIntrinsic() which sets a hidden property on an
  object which is used as a hint to the rest of the engine that
  (a) the value doesn't need to be resolved anymore and (b) we can
  use it as a type hint to reject non-intrinsic complex types during
  stringification. A wrapper class would have been nicer but this
  entails fewer code changes (deepEqual everywhere...). A convenience
  class has been introduced in the CloudFormation section to make
  impact minimal.
- Implement inversion-of-control on the FnConcat thing, execution
  engines (name?) can now register themselves to handle string
  combining. Still not perfect because if there are not intrinsics
  we can't really guess the engine (so we "default" to CFN anyway)
  but we have the initial trappings of CFN decoupling.

ALSO
- Add a run config and a helper script to debug a unit test from VSCode
  (breakpoints \o/)

I expect this to generate a healthy amount of debate. Also I would
appreciate suggestions on naming of the various pieces, I'm afraid
I haven't been 100% consistent just yet.

- What do we call CloudFormation/Terraform/...?
- What do we call stringified Tokens?

Changed tokenAwareJsonify() -> TokenJSON.stringify() to mimic
JSON.stringify() API name. Implementation does not depend on override
of toJSON() anymore, because there's no way to do that correctly.
Instead, the function has a custom implementation.

Split new code over more files to separate them out.

…-TypeScript languages anyway

…ngification

Fix an issue where the wrong resolution method was used to resolve policy documents.

…on-intrinsics

… cross-account-destination policy usage

… engines

To avoid an undue number of {Fn::Join}s in the output, add an optimization
into FnConcat() to inline nested FnConcats.

…ngification