Colander: Support schema binding.

This commit adds support for Colander schema/deferred binding by binding the
active request. This causes any deferred values to be resolved, tripping
validation or other custom logic during deserialization.

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My use case for this addition is to validate i.e., PUT, POST, etc. operations on entities by ensuring that the requester "owns" the object they're trying to modify. Rather than using dynamic ACLs or some unrelated method for this, it's nice to be able to stick with Colander.