Blockpc

This patch addresses issue #81 by implementing blockpc based on Peter's
suggestion. blockpc has the same semantics as pc, except that pc only applies
when the given predecessor of the given block is chosen.

blockpc(s) are collected in a way similar to what we currently do for pc(s),
and are encoded with a similar technique for encoding UB constraints.

The parser is also updated to parse blockpc(s).

I am sorry the path is a bit big...

I think the parser/printer should follow an old rule from networking: "Be conservative in what you send, be liberal in what you accept". 

In other words, it's perfectly fine if there are restrictions in the printer that do not have corresponding restrictions in the parser.