Fix encoding of high surrogate in UTF-16

[dra27]

Found while torturing AppVeyor by compiling OCaml in a directory called 🐫

[dra27]

I'll review #46 tomorrow so that that could be included too, but I think there should be a release pre-4.06.0 containing this fix (it'd be hard to attack, but it feels wrong for OCaml to depend on a program with a faulty Unicode conversion)

[nojb]

Unrelated to this PR of course, but the UTF-16 encoding could be done with Buffer. add_utf_16le_uchar from 4.06 onwards and we could add the present code to the Compat module.

[dra27]

Actually, that's a much better idea - I'll add that to tomorrow's train-list!

[dra27]

And three more commits to do that:

• I found on my local AppVeyor an issue with the compiler updater (flexdll bootstrap didn't used to work if you had flexlink in PATH with no flexdll.h). I can separate that, but I don't expect you're worried about that being here!
• A "slower" version of Buffer.add_utf_16le_uchar is added to the compatibility layer for pre-4.06.0 builds
• A very simple Uchar module is added to the compatibility layer for pre-4.03.0 builds (I haven't actually tested that, but I think from inspection that should be OK)

[alainfrisch]

@dra27 Do you want to give a look at #46, or do you prefer me to create a release now?

[dra27]

Sorry - I had, but I see that I was clearly expecting you to know my thoughts psychically! I'll go and commit them now...