Make the `computations` and `extensions` namespaces singular.
Split the intrinsics and add the `ComputationName` concept.
Make `dependencies_of` and `name_of` intrinsics.
Update mpl_extensions.
Add the `extern_` computation.
Update the captain's log.
Add the `checked` computation.
Ignore unprovided computations in the `dependency_graph`.
Major cleanup:

* Replace the `augment` intrinsic by the `bind` intrinsic
* Replace the `update` intrinsic by the `execute` intrinsic
* Do not fold computations when they are `executed`
* Remove computations made irrelevant by those changes