Add no_std support w/ alloc & embedded-io

[i404788]

This is a pull-request for those interested not necessarily for upstreaming (see #794 (comment)).

It uses a custom esp-idf-part which is no_std but alloc (https://github.com/i404788/esp-idf-part).

Main changes:

• Move command outside of connection
• Add embedded-io with adapters to keep existing functionality intact
• Replace HashMaps with BTreeMaps (adds Ord)
• Make all dependencies also available without std

Importantly it's not 'no-alloc` because that is very hard to maintain with the std counterpart.