[m-rev.] for review: Speed up retrieval of foreign imports.

[Peter Wang]

On Tue, 22 Nov 2022 15:57:40 +1100 Peter Wang <novalazy at gmail.com> wrote:
> 
> This change replaces the representation of cached_foreign_imports from
> tree234 to one based on version_array. It reduces the average run time
> for a do-nothing build of Prince on my machine from 7.850 s to 6.505 s,
> for a speedup of ~17%.
> 
> compiler/va_map.m:
>     Add a module originally written for Prince.

Actually the 2-level scheme employed by va_map isn't necessary for
this use case. I will try using a flat version_array.

Peter