This removes the last use of `strdup`
This required making a lot of related pointers be `const`.
That in turn conflicted with the need to `munmap()` a pointer
eventually, which was similar to the need to eventually `free()`
an `Expansion`'s contents, so I used the same solution of a
`union`. That lets us normally use the `const` pointer for
`const` correctness, and the non-`const` one for the not-really-
mutating destruction cases.
* Replace FAMs with `std::vector`s (or one `std::string`) in four `struct`s
* Anonymous types declared in an anonymous union are also non-standard
Only Clang complains about this (-Wnested-anon-types)
