Zlevels field function update

[Coolthulhu]

Includes #11445

Changes map functions related to fields to tripoints. Old invocations are handled by overloads. Overloads construct tripoints.
As far as I know, most of the performance penalty from field comes from fires iterating over all items. If this is not true and fields should retain all their performance, I'll add back the 2D functions in their current form.

For the often-used functions (such as has_flag) I keep their original functions for now, with the tripoint overloads being totally separate. This wouldn't be very maintainable in long term, but this is just a temporary measure - I intend to first create all the overloads for map functions, then remove the invocations of their 2D versions, then finally remove the 2D versions totally.

[Zireael07]

Can't wait to see it in action!

[Coolthulhu]

Sorry to disappoint, but this is purely a technical change. Actual z-level fields like tall fires and clouds of smoke require this change, but are themselves a separate thing.

The entire first "batch" of PRs is just technical stuff.

[Zireael07]

Still, that first batch is necessary for clouds of smoke to happen :)