add extensive documentation on the execution model

implement some of the earliest Forth loading stuff, following Jonesforth closely

the Forth memory space isn't totally set up yet - we are going to use the data segment after all, though it won't be heap-ish really. everything up through _start is ready for Forthy-y use though, and it does `cld` besides.

Force-Push: yeah
Change-Id: Ibd12223f304aff71f4e78744f7a9da09b9072a45

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