async support with feature

[tommasoclini]

add async processor support using maybe_async_cfg2 on feature "async".

I tried running memory.sh and I'm getting weird results, there are 200 more bytes in every measure in every test, but when comparing actual binary size with cargo size or avr-size, the main branch and this branch are equal.
I used this command(avr-objcopy -O ihex binary.elf binary.hex) to get the raw binary, and the two files(old and new) do not differ.

[tommasoclini]

@Finomnis what do you think?

[Finomnis]

Features should enable capabilities but they should not remove them (like what you do with the original sync function)

Reason is that other libraries might enable features out of your control.

But that's a general guideline, if you say this crate is enduser specific, then maybe this might be a solution, yes.