Allow gaps in RIOdesc

[oddcoder]

That will be useful for plugins like the ihex one which allows having gaps in the physical address space.

[condret]

Can't you just force the plugin to fill such gaps with data configured in RIO? In r2 i did the treebuf plugin with does copy on write over the entire addr space, backed by a rbtree. Ofc there are gaps in the desc, but these get filled on read by the plugin with io->ff

[oddcoder]

I don't remember what I was doing in 2019 :P