Sequence: Support nested and indirect lists for db / dw / dd.
Adds a sequence type for matching. Not used it in Expression getHead / getTail
yet because those are used in parameter matching where it doesn’t work yet.
Identifier: Remove exists check for isRegister, isFlag and isGroup.
No longer needed since Context.getSymbol returns the bottom type now which
evaluates true for every type, rather than throwing an exception.