Allow bitflags v1 and v2 to coexist

[manushT]

The patch breaks cargo's additive features

Fixes #877.

[jonathanpallant]

Unfortunately this doesn't work because Library A might use defmt to describe some data type and expect the bitflags v1 API whilst Library B might use defmt but expect the bitflags v2 API (if we add it).

You could have a feature flag for bitflags v2 so that we keep the dependency list as short as possible, but it has to be additional and exported as a different macro with a different name.

[jonathanpallant]

The CHANGELOG was reformatted. Can you rebase?

[jonathanpallant]

The format check failed, I'm afraid.

[jonathanpallant]

@manushT - we still have an outstanding comment on this one.

[jonathanpallant]

There's still an outstanding question around importing and re-exporting the bitflags API so that people don't have to try and include the same version of bitflags in their application that defmt was using. I think that's a good idea.