update agenda and readme

document boxed data storage and sequences
comment quotation defining word
resurrect forward compilation primitives
inline data type notes
agenda update regarding better nested sequence implementation

Directly advancing DP while accumulating is not a good idea since
nested calls (e.g. to parsing words) will completely override current
sequence accumulation state.  Better: accumulate everything on stack,
leaving total size on top.  That way, following compilation function
can decide memory operations, including flexibility in header layout
agenda: graphical decompiler
receive stack effect
2swap shuffler
clarify until-tokens purpose
quotation reader rework, not working yet

stack underflows, weird!