Provides very flexible arena type Arena<P, T> and pointer type Ptr. The arena supports
non-Clone T, optional generation counters (zero-cost when omitted), and zero-cost arena
differentiation. The generic P in P is a marker that can be used to guard against using the
wrong Ptr in the wrong Arena when multiple Arenas are in use. The P can optionally include a
generation number, in which the arena will use a generation counter to prevent invalidated pointers
from working. No unsafe is used. no_std compatible.
Also includes SurjectArena<P, T> as a capable new kind of union-find data structure, and
ChainArena<P, T> as an idealized doubly-linked-list-on-an-arena structure