Added "hand" and "pos1" pseudo block types that use the current quickbar slot and the block at the current pos1, respectively.

Work in progress.
Just putting it here because it entails some API additions someone might object to.

Still missing:
~~- A translation table for the placeable items~~

Discussion about item->block mappings:
- Currently, mapped items have a wildcard data value (-1), which, I guess, affects //replace (They're replaced by 0 as a last resort before reaching things like //set). Do we want it that way or not?
- If so, do we want a wildcard data value for blocks whose data value does *not* depend on the damage value, too? (i.e. stairs)
- If so, do we want a wildcard data value for the remaining blocks (i.e. logs), too, if their damage value is 0 (or in general)?
- If not, how do we tell the two types apart?