multiple definition of malloc, calloc, and free

[noahbliss]

Cross-posting this issue: https://github.com/esp-rs/esp-wifi/issues/438

I use another crate which wraps some C code. That crate and this crate both rely on the C functions listed in the title. When both crates are used, the compiler dies of an aneurysm. Would it make sense to make a default-enabled feature which enables these functions inside the crate, but that users can opt-out of if they have implemented them inside their own parent project?