A prefix system, used with PrefixedUnit.
Use the DefinePrefixSystem mixin to automatically generate a helper function like kilo() for every prefix in the system.
getPrefixes has to be a parameterless callable returning Prefix. This scheme is used (instead of directly passing the prefix array as a parameter) to reduce code bloat, because delegate literals are mangled much shorter than array literals. The effect on the binary size is quite strong because the mangled name of a PrefixSystem instance is part of every symbol in which a PrefixedUnit is involved.