Change the mainloop so that words are executed immediately after they're read.
This greatly simplifies execution model and allow the "DEFINE" word to become
an IMMEDIATE and stop its "copy from compiled words" scheme.
The downside to this is that flow control words no longer work when being used
directly in the input buffer. They only work as part of a definition.
It also broke "RECURSE", but I've replaced it with "BEGIN" and "AGAIN".
Another effect of this change is that definitions can now span multiple lines.
All in all, it feels good to get rid of that COMPBUF...
Readline, instead of being triggered by the end of execution of the last
compiled line is now triggered "just in time", by "WORD".
This allows IMMEDIATE words reading input buffer to span multiple lines
( comments for example, but colon definitions will soon follow ).
This will allow us to support backward branching with just one new (bbr) word.
Also, this allow us to have "(" word sooned in core.fth and thus allow for
earlier commenting.
Big one.
This allows us to write higher order words directly in Forth, which is much
more convenient than writing post-immediate (see "NOT" structure in diff if
you want to see what I mean) structures in ASM.
These structures can then be written to ROM (rather than loaded in RAM for
definitions loaded at run-time).
That's quite a bit of tooling that was added, 2 compilations stages, but I
think it's well worth it.
