Add valueToLiteral()
* Adds `valueToLiteral()` which takes an external value and translates it to a literal, allowing for custom scalars to define this behavior.
This also adds important changes to Input Coercion, especially for custom scalars:
* The value provided to `parseLiteral` is now `ConstValueNode` and the second `variables` argument has been removed. For all built-in scalars this has no effect, but any custom scalars which use complex literals no longer need to do variable reconciliation manually (in fact most do not -- this has been an easy subtle bug to miss).
This behavior is possible with the addition of `replaceASTVariables`
