Attempting to specify the format

[Yoric]

Here's a first attempt at specifying the format.

@dominiccooney, did I get it right?

[arai-a]

haven't yet done Nodes and probabilities part.
will review shortly.

[arai-a]

how about adding something like the following?
(just suggestion for later changes, not that they should be added in this PR)

• the dictionary defines the possible AST, and code for each possibility to encode the actual tree in tree part of the file
• the possible AST is the subset of the valid ECMAScript AST
• If there's only one possibility at some node, it's fully defined in the dictionary
• if there's 2 or more possibilities at some node, code in tree part defines which possibility to choose at that node
• primitives values (excluding strings) and the length of lists are all defined in this dictionary, not in tree part

[Yoric]

I've tried clarifying in the latest version.