document (or write) semantic analysis passes

[driusan]

A lot of open issues such as #54 and #36 require knowledge of the global semantics of a program that go deeper than the syntax of PHP or VM opcodes. It would be useful to document what mechanisms are in place (or what architecture should be) to handle things like "determine if this array is a fixed length with numeric keys and all the same type".

[ircmaxell]

I'll take a few passes at it. I've been hesitant due to the flux the project has been under, and I feared docs going out of date quite quickly.

But I do think it's time. I'll work on a few blog posts about that.