Add lazy propagation to treaps

[Matistjati]

KACTL's current treap implementation lacks lazy propagation (IMO, quite an important feature). Adding this is nontrivial if you don't understand the implementation. I chose to add reverse interval, as it is not uncommon, nontrivial and requires little code.

In order to reduce code size, I implemented the treap as described in cp-algorithms.

Some stats:

• New treap with reverse: 61 LOC counting comments. ~1308 characters excluding comments
• Old treap: 54 LOC. ~1119 chars
• New treap without reverse or lazy propagation: 42 LOC. ~980 chars

Runs in 0.74 on substring reversals. code

Runs in 0.66 on cut and paste if we don't push, code. 0.73 if we do push.
Runs in 0.65 with the old implementation.

So performance seems to be basically the same. It may be harder to understand because of return-by-reference, but I think it should be understandable by the comments and sample usages.

If you use the treap as a set, merge has a precondition that max_val(l) < min_val(r). Should we use a line to include that comment?

If you want, I can codegolf away 2 lines related to rev without hurting readability too much.