Replace subparser tree with struct tree

[shinzlet]

Currently, Phreak's core code implements a tree of subparser objects, where code is recursively executed until an endpoint is reached (there are no more expected keywords). However, it might be more memory and time efficient to instead build up a tree of binding structs, then iteratively walk through that tree.

Worth considering, but not a priority. Would require major rewriting.

[shinzlet]

I'm becoming unsure wether this would be possible (or reasonable) to pull off, but it's definitely not happening for a looong time even if it is!