The CurrencyRateController has been migrated to the BaseControllerV2
API, which includes various API changes. These changes include:
* The constructor now expects to be passed a
`RestrictedControllerMessenger`.
* State changes are subscribed to via the `ControllerMessenger` now,
rather than via a `subscribe` function.
* The state and configration are passed in as one "options" object,
rather than as two separate parameters
* The polling needs to be started explicitly by calling `start`. It
can be stopped and started on-demand now as well.
* Changing the current currency or native currency will now throw an
error if we fail to update the conversion rate.
The `ComposableObservableStore` has been updated to accomodate these
new types of controllers. The constructor has been updated to use an
options bag pattern as well, to make the addition of the new required
`controllerMessenger` parameter a bit less unweildly.
The `assert` module has two modes: "Legacy" and "strict". When using
strict mode, the "strict" version of each assertion method is implied.
Whereas in legacy mode, by default it will use the deprecated, "loose"
version of each assertion.
We now use strict mode everywhere. A few tests required updates where
they were asserting the wrong thing, and it was passing beforehand due
to the loose matching.
