Document public AST API for macros

[andreyvit]

I understand that we're in 0.x stage, but would be nice to get at least a rough idea of which AST methods/props are going to become the public API is the future (i.e.: are safe to use inside the macros).

[ckknight]

As of 0.9.x, the macro API is mostly stable. I do need to document all the methods and such, though.

I also want to add support for async macros and such before I deem it fully stable (at which point I'd be getting close to a 1.0 release)

[andreyvit]

So you mean all that stuff accessed from the prelude is going to become a public API after 1.0? With a commitment to backward compatibility?

[ckknight]

You are correct.

There are a few minor naming things I want to change (e.g. cacheable to is-cacheable), and adding more documentation to each part involved, but other than that, it will be stable.