get_comb_opcodes originally stood for "get combined [pseudo- and evm-]
opcodes". what it really meant is get all LLL opcodes.

this commit forces stack items passed to labels to be explicitly named.
this will help liveness analysis of stack items. note that it changes
the structure of labels - instead of allowing labels to occur
arbitrarily in code, they turn into blocks, enforcing scope of the named
stack items.

This reverts commit eca642f.

This reverts commit a73500a.

cases:
- internal void
- internal with return
- external void
- external with return

Change labels in LLL to be parameterized. Previously, stack items were
implicitly consumed from jumpdests using `pass`, which made liveness
analysis for stack items impossible in general. Since labels now take
parameters, they are very similar to subroutines so we could use that
terminology interchangeably.

This change also enables translation to other architectures such as LLVM
backends since implicit stack items cannot be handled in translation to
a register machine.

Lastly, this commit also added an `exit_to` instruction for LLL. This
compiles the same as `goto` -- to a JUMP instruction -- but has
different scoping semantics. `exit_to` terminates a subroutine while
`goto` expects control flow to be returned. This is an important hint
for liveness analysis. If you hit an `exit_to`, all stack items in scope
must be consumed, whereas if you hit a `goto` the same requirement does
not hold.

In order to accomodate the semantic change, this commit changes the
codegen everywhere that stack items are consumed implicitly. In
practice, this happens in private functions, and in function common
cleanup blocks.

Note: the assembly output for all contracts I looked at were
byte-for-byte the same with the exception of those which returned
straight out of a loop (using the LLL `cleanup_repeat` instruction), so
this commit adds a couple more tests to test returns out of loops (in
the void return and external cases)