strengthen compile_decs_thm by only requiring handler for exceptions

i.e. if the semantics isn't going to produce an exception, the machine
doesn't have to have the handler installed. however, the theorem still
needs a handler for timeout exceptions even though they shouldn't be
necessary, because compile_val is stated weakly (and i don't want to go
touching it right now..)
rework statement of compile_val

went into it anyway... now it only requires the stack to contain a
handler if it needs to raise a handled exception (and only requires a
return pointer if it needs to return).
update proofs for improved compile_val, and prove compile_dec_divergence

the one dec divergence is relatively easy... the more finicky one is
divergence somewhere in a series of multiple expressions... but probably
just a matter of working through an induction..