re_eval and closures: add lots of TODO tests

re_eval and closures: add lots of TODO tests

re_evals currently almost always do the wrong thing as regards what
lexical variable they refer to. This commit adds lots of TODO tests that
show what behaviour I think there should be. Note that because hardly any
of these tests pass yet, I haven't been able to verify whether they have
any subtle typos etc.

The basic philosophy behind these tests is:

* literal code is compiled once at compile-time and shares the same
  lexical environment as its surroundings; i.e.

    /A(?{..$x..})B/

  is like

    /A/ && do {..$x..} && /B/

* qr is treated as a closure: compiling once, but capturing its
  environment anew each time it is instantiated; i.e.

    for my $x (...) { push @r, qr/A(?{..$x..}B)/ }

  is like

    for my $x (...) { push @r, sub { /A/ && do {..$x..} && /B/ } }

* run-time code is recompiled each time the regex is compiled; literal
  code in the same expression isn't recompiled; i.e.

    $code = '(?{ BEGIN{$y++} })';
    for (1..3) { /(?{ BEGIN{$x++}})$code/ }
    # x==1, y==3

* an embedded qr is not stringified, so the qr retains its original
  lexical environment; i.e.

  $x = 1;
  { my $x = 2: $r = qr/(??{$x})/ }
  /A$r/; # matches A2, not A1