what Yapsi needs for first-class closures

gistfile1.txt

# block 0
my $c;
{ # block 1
  my $a = 5;
  $c = { say $a } # block 2
}

$c()

We want the above to print "5\n".

Here's the thing:

* OUTER:: is the block that syntactically surrounds the current block. For
  example, the OUTER:: of block 1 is block 0, and the OUTER:: of block 2 is
  block 1.

* CALLER:: is the block that called the current block. So block 0 is the
  CALLER:: of block 1, but in block 1 we don't call block 2, we just store
  it in a variable.

Instead, block 0 is the CALLER:: of block 2. Note that $a has "gone out of
scope" by the time we call $c(), but it's still retained because $c has a
reference (through block 2's OUTER::) to $a's lexpad.

In Yapsi, there's the concept of OUTER:: through the LexPad objects. Each lexpad
knows about its outer lexpad. There's no notion of CALLER:: yet. We need that for
dynamic variables to work, so we might as well add that now.

Also, the last time I tried to make the above code work, I ran into problems because
blocks are too special-cased in the compiler. That probably needs some looking-over
for the above to ever work.