Previously, inventory iterators could only be used with non-const
objects, leading to suboptimal const correctness.
Adds support for `const` objects to inventory iterators via
C++17 CTAD.
Introduces new functions to check for presence of and remove player items.
These functions do not rely on or expose item indices.
They're implemented as free functions instead of Player methods due to
complicated include dependencies between `player.h`, `inv.h`, and
`inv_iterators.h`. We should probably look into cleaning this up at some
point.
Gleb Mazovetskiy
Juliano Leal Goncalves