When reading config options from files, it's important to make sure the
value makes sense - the incoming data could be anything, and may have
incompatible values or be otherwise unreliable.
This introduces a simple way to define and use "mapping tables" which
can map between a "native" value, probably an enum (but not necessarily)
and some external value, for example the value written to file.
This provides a decoupling between the two values, as well as sanitising
inputs and outputs. This is important, as over time, if there isn't a
decoupled interface, changing options result in corrupt configs, or a
proliferation of obsolete enum values kept for compatibility.
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