Adding Appveyor CI

[elichai]

Hi,
After a bunch of discussions over time on windows and travis,
which is pretty buggy because it's still considered beta.
#147 #148 #84

This Will introduce a separate CI for native windows.
This will check the following:
beta, stable, nightly, 1.22.0
with targets: MSVC,GNU each one 64bit+32bit
emscripten for stable
and test that the benchmarks compile on nightly.

this works with caching.

The big caveat is maintaining two different CIs.
I think it can be partially mitigated by separating the tests themselves into a separate file (though i'm not sure about this file supporting both windows and linux)

but with this we could potentially remove the windows builds from travis and have this in parallel

[elichai]

A test sample:
https://ci.appveyor.com/project/elichai/rust-secp256k1/builds/26829109

[real-or-random]

Nice. MSVC and GNU together is certainly useful. Actually, does Rust on Windows on Travis use MSVC or GNU? I think it's MSVC but I don't know to be honest. Could we enable both on Travis too?

I think it's in general a good idea to have a script that runs all the tests (or one for each OS, even if
they look very similar). This makes it easier to run extensive tests locally.

By the way, I just found this: https://github.com/japaric/trust Maybe it's useful.
Also this is interesting to know: rust-lang/rustup#998 (comment)

[elichai]

I don't know either, although I think it's GNU because Travis somehow accepts GNU like commands (like the ifs in our travis config aren't failing on windows)

[elichai]

@real-or-random Apparently I was wrong. https://travis-ci.org/rust-bitcoin/rust-secp256k1/jobs/574387376#L33

[apoelstra]

I think we're more likely to move to github CI than to appveyor.