update the representation of the block instruction

We have changed the block's representation to "block n", so
update the relevant part in the documentation.
Merge pull request #118 from chenyang78/block_doc

update the representation of the block instruction
fix use after free bug
Merge pull request #120 from regehr/fix

fix use after free bug
Merge remote-tracking branch 'upstream/master' into iN-synthesis-noinfer