MultiPV support

[Ciekce]

Design essentially ported from Stormphrax. Behaviour matches SP's exactly (barring any bugs I've missed).

Marked as draft because no test has been run, and there are implementation decisions from SP that I am unsure are wanted in CW:

• when the UCI option MultiPV is set to 1 (i.e. multipv disabled), the multipv N output in info strings is suppressed
• In the (ideally impossible) scenario that a root move could not be found, std::terminate() is called

Test  | multipv
Elo   | -1.43 +- 1.12 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=16MB
LLR   | -2.97 (-2.94, 2.94) [-3.00, 0.00]
Games | N: 123120 W: 31899 L: 32404 D: 58817
Penta | [1485, 14854, 29314, 14495, 1412]

https://ob.cwchess.org/test/1528/

Almost all the machinery for a future working TB implementation is also present. The root move implementation allows for some other cleanup (notably moving node counts from the enormous table into root moves), but that is out of the scope of this PR. "Verbose MultiPV" as I call it (printing info lines after every single PV is searched, after a delay) is also omitted, as it requires some somewhat involved refactoring of how/when info lines are printed.

A side effect of this implementation is that partial depths are now accepted (safely) if search stops mid-iteration.

[TheRealGioviok]

* when the UCI option `MultiPV` is set to 1 (i.e. multipv disabled), the `multipv N` output in info strings is suppressed

* In the (ideally impossible) scenario that a root move could not be found, `std::terminate()` is called

lgtm, first one probably good to avoid problems with some guis

[Aethdv]

lgtm