lib: Introduce Cross Index concept

A Cross Index, short for Cross Platform Pair Index, is the essential
shape of a splice, without the invoking the more mind bending concept
of adding variations of for these 6 pairings to an existing thing so
that it can be switched out for something else.

So the purpose of a Cross Index is to contain the result of `f`\
(which may be reified in code, or just an abstract concept):
 - f "build" "build"
 - f "build" "host"
 - ...

Splicing on the other hand refers not just to these six variants, but
to the idea of tacking them onto one of the variants. (hostTarget,
I believe)

Cross Indexes are a necessity for making cross compilation work, but
splicing is more than necessary.

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