[perl #77452] Deparse BEGIN blocks in the right place

[perl #77452] Deparse BEGIN blocks in the right place

In the op tree, a statement consists of a nextstate/dbstate op (of
class cop) followed by the contents of the statement.  This cop is
created after the statement has been parsed.  So if you have nested
statements, the outermost statement has the highest sequence number
(cop_seq).  Every sub (including BEGIN blocks) has a sequence number
indicating where it occurs in its containing sub.

So

 BEGIN { } #1
 # seq 2
 {
   # seq 1
   ...
 }

is indistinguishable from

 # seq 2
 {
   BEGIN { } #1
   # seq 1
   ...
 }

because the sequence number of the BEGIN block is 1 in both examples.

By reserving a sequence number at the start of every block and using
it once the block has finished parsing, we can do this:

 BEGIN { } #1
 # seq 1
 {
   # seq 2
   ...
 }

 # seq 1
 {
   BEGIN { } #2
   # seq 2
   ...
 }

and now B::Deparse can tell where to put the blocks.

PL_compiling.cop_seq was unused, so this is where I am stashing
the pending sequence number.