0.12 compatibility

[Snektron]

It would be nice to have both master and 0.12 compatibility. This allows us to more easily apply new changes, and to keep 0.12 in CI. This can be done by comparing with @import("builtin").zig_version.

I don't think it makes sense to get compatibility with ALL development versions released in the meantime, so I think we'll only need to support latest master and latest stable.

Tasks:

• Create some "compatibility" layer. Ideally, we only need to put a few things in it. Im thinking compat.zig or something where we just re-export renamed functions and write small wrappers. We'll see how well it goes.
• Set up CI for 0.12 in addition to current master.
• Update readme with explanation about new flow.

Supersedes #124

[Snektron]

@slimsag Would you be interested in maintaining compatibility with Mach nominated versions? It wouldn't be that much effort to add, but then there needs to be some infrastructure for keeping that up to date.