terminology rename; "StackItems" is confusing because it's unclear whether it refers to the width of the item or the width of the index needed to address them. We select "StackDepth" because that's a bit more clear, hopefully.

redoing the table/stack Layouts to just take the language parameters; also fixed bug where we were mistakenly indexing by nonterminals and not reduce rules

start working on downstream transaction flow control (the case where we present data but downstream isn't ready yet)

test using the internal nMigen "Part" function from nmigen.hdl.ast to see if we can do variable-width slicing but no alas it fails