Switch CI to Azure Pipelines

This commit switches CI for the wasm-bindgen repository from a mixture
of Travis and AppVeyor to Azure Pipelines. One of the main reasons for
this was for me to personally get familiar with Azure Pipelines, but
after finishing it I think that this'd be a good transition for this
repository anyway.

With azure pipelines we're able to get more parallelism (10 vs the 3 on
Travis plus 1 on AppVeyor) as well as house all configuration in the
same location for one service (Azure Pipelines covers all 3 major
platforms).

I've tested this a good deal on my own personal repository as well, and
I believe that this is a lossless representation of our current CI,
releases and all!

Author: alexcrichton