Can you add a way to calculate the max heap used?

[patnir1]

At the moment you provide heap.used() but i need something like heap.maxUsed() to validate some of the code that we are working with can work on certain device types

[newAM]

This could be done in this crate, but it would make more sense to add them in the underlying alloc crates.

[dragonnn]

This can be done with trallocator https://crates.io/crates/trallocator as it can easily wrap embedded-alloc.