Restore building on very old OCamls

[dra27]

I intend to overhaul this to switch to GitHub Actions, as we'll get much better parallelism, but in the meantime.

I do not propose that we actively support building flexlink on OCaml 3.11, but it is quite useful that it basically still does (especially when opam finally allows installing the old Windows compilers in switches). I propose that dropping support for building flexlink with very old OCamls should be done consciously as part of a larger - long overdue - overhaul of the code, not just because of little patches.

With that in mind:

• restoring 4.02 support involves a slight hack to the Compat system. As part of #85, I will switch this to be rather better pre-processed, but for now it just involves moving Buffer.to_bytes into the Compat406 and not caring that it gets redefined for OCaml 4.03-4.05 when it doesn't need to be
• Restoring 3.11-4.01 support involves converting a single match on an exception back to try

The AppVeyor matrix is then expanded - I debated getting it to skip most versions for PRs, but I propose just putting this in as-is for now, given that the traffic on this repo I so low...

[dra27]

Note that while the first time run is very slow for this, caching makes the AppVeyor run much quicker (see this run on my fork). As far as I can tell, when I (or someone) switches this to a well-engineered GitHub Actions workflow, we'd get a lot of parallelism for free.