Since Go doesn't support unions, and because the translation format is a bit
wacky with the same field having multiple types, we must resort to
introspection to switch between single-item translation (for singular), and
multi-items ones (for plurals).
Previously, introspection was done at runtime, which is not only slow, but will
also only catch typing errors while trying to use the translations. The current
approach is to use a struct with a different field per possible type, and
implement a custom unmarshaller to dispatch the translations to the right one.
This should marginally reduce the memory consumption since interface-boxing
doesn't happen anymore, speed up the translations matching, and enforce proper
typing earlier. This also allows us to remove a bunch of now-useless tests.
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