handler push and pop

[Stephen Weeks]

I just committed the changes for putting HandlerPush and Pop back in
the SSA IL.  From the commit log:

Added HandlerPush and HandlerPop to the SSA IL.  These will now be used
throughout the SSA simplify pipeline, and at the very end, a new pass,
implementHandlers, will translate them into SetHandler and friends.  So, all the
SSA simplification passes only see HandlerPush and Pop and the backend only sees
SetHandler and friends.  For now, implementHandlers uses HandlerPush and Pop as
"hints" to decide where to put SetHandler and friends.  But in the future, it
will ignore them and use other strategies, to appear in our exceptions paper.

Also changed the Raise transfer so that it only takes one argument instead of a
vector.  We never used the generality, and the backend assumes that there is
only one anyways.