Execute a simple script block.
Remove terms `_statement_flow_control_statement` and `_statement_pipeline` from grammar

Since they don't really do anything.
Implement `return` for scriptblocks. Now this works:

    PASH> & { return ; 1 } ; 2
    2

I used an exception to abort the current script block. I'd rather avoid
using a exceptions in normal program flow, but I didn't see a
straightforward way to handle the way `return` can pop so much execution
state.

One downside is that the AST Visitor dispatcher, which uses reflection to
invoke the handler, unwraps and rethrows the exception. Ugly.
Merge pull request #97 from JayBazuzi/return

execute a script block, and `return`