This no longer accepts incoming variable values in a potentially lossy way, mirroring the existing behavior for literals. This fixes an issue with GraphQL.js being not spec compliant.

This is breaking since servers which used to accept incorrect variable values will now return errors to clients.

Serialization of values is not affected in the same way, since this is not a client-visible behavior.

As a bonus, this adds unique serialization and coercion functions for the ID type, allowing to be more restrictive on numeric types and un-stringable object types, while directly supporting valueOf() methods (ala MongoDB). The changes to how the ID type serializes and coerces data could be potentially breaking.

Fixes #1324

…gic to string serialization