Stop bundling a global allocator

[TheButlah]

Looks like esp-wifi includes a global allocator. It would be better to let the user choose their own global allocator.

I'll submit a PR to do this.

[fenduru]

This also prevents me from using esp-wifi in a project freshly created with https://github.com/esp-rs/esp-idf-template as both the std and no-std options there bring in an allocator

[MabezDev]

This also prevents me from using esp-wifi in a project freshly created with https://github.com/esp-rs/esp-idf-template as both the std and no-std options there bring in an allocator

@fenduru if you are using an esp-idf based project, you do not need esp-wifi. The esp-idf-* packages already cover WiFi usage.

[fenduru]

@MabezDev they don't cover BLE

EDIT: On second thought I don't even need the IDF for my use case so I'm going to switch to the esp-template instead, but it does still seem like global allocator initialization is something that the final binary should be doing rather than a library (since there can only be one)