Tokens are now represented by the interface `IToken`. The class `Token`
still exists, but it is used to hold static routines for token encoding
(`Token.encodeAsString()`), and as a potential base class for `IToken`
implementations so some default functionality is inherited (this
inheritance is an implementation optimization, it is not a
requirement!).

Actual token use is split into 2 categories:

- *Intrinsics*, represented by `Intrinsic` class: these are to represent values
  that will be understood by the deployment language formalism (e.g.
  CloudFormation), and can be used to escape language type checking.
- *Lazy values*, represented by a `Lazy` class with static factories:
  these will be used to represent type-checked but lazily-produced
  values (evaluated at synthesis time).

In order to be JSII-compatible (which does not currently support
lambdas), `Lazy.stringValue()` et. al. take an interface with a single
method instead of a function.

Also changed in this commit: shoring up property names for encoded
tokens in classes like `CfnParameter` and `CfnElement`.

* `.stringValue` => `.valueAsString`, etc so `.value`, `.valueAsString`,
  `.valueAsList` etc. group together in autocomplete.
* `.ref` now returns a Token, `.refAsString` returns the stringified
  version that token.

To match the other standard the latter should actually be `.refAsString`
but `.ref` is used in so many places that this might be uncomfortable?

Revert `Token.isToken()` to check if the passed object is literally an
`IToken` (to match `Array.isArray()` and other related methods), and
`Token.unresolved()` to check if the passed object might also be an
encoded Token.

Fixes #1933.

BREAKING CHANGES:

* `Token` can no longer be instantiated. Instead, instantiate
  `Intrinsic` or use `Lazy.stringValue` and others.
* `Token.isToken()` will only return true if the object is a Token,
  for detecting arbitrary encoded tokens, use `Token.unresolved()`.
* `CfnOutput`: remove `.ref`, since they can't be referenced anyway.
* `CfnParameter`: rename `.stringValue` => `.valueAsString` (and
  similar for lists and numbers).
* `.ref` now returns an `IToken`, use `.refAsString` to get a
  string-encoded Token.