re_op_compile(): rejig code-block handling code

re_op_compile(): rejig code-block handling code

In the code that keeps track of the start and end positions of each
code block, i.e. (?{....}), refactor it:

Rather than an array of STRLENs, with two slots to mark start and
positions, turn it into an array of structs; each holding the start and end
position, plus a pointer to the code block op. This will make it easier
when we shortly allow literal code from run-time patterns too.

Also rename some of the variables accordingly.