This makes it simpler, from the programmer's perspective, to get the
name of an axis as a string slice and to format an axis name into a
string. To me, the matching method `Axis::name` seems more direct than
the explicit lookup table that it replaces, and I'm hoping that it will
be about as easy for the compiler to inline, or even easier.
Implementing `Display` enables us to hand an `Axis` to a string
formatter without any explicit conversion. It adds extra code in the
short run, but I'd expect it to simplify our code in the long run by
fitting into the conventions set by the Rust standard library.
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