MarkerAllocator::maintain is never called #713

Description

Apologies if I've overlooked something, but AFAICT, nothing ever calls this trait method.
Is this something that I need to setup for myself?
I've been using https://specs.amethyst.rs/docs/tutorials/13_saveload.html as a reference to
use with my own allocator impl, and that doesn't give guidance on how maintain is called.

Meta

Rust version: rustc 1.48.0 (7eac88abb 2020-11-16)
Specs version / commit: 0.16 but I also looked through 1fd81cd and didn't find any callers
Operating system: N/A, but I'm on Linux.

Reproduction

$ cd specs
$ grep -r maintain .
<none of the results show maintain being called with any arguments>

Expected behavior

Either something should call maintain inside specs, or the documentation should explain how best to perform that maintenance!