Remove most `repr(C)`

[chrysn]

Many internal types are repr(C). This is probably a relic from before the C bindings started copying everything into own structs that then are repr(C) -- but otherwise we don't do anything to warrant their use, so that repr can go away. (This is a breaking change, since repr(C) is an unsafety guarantee for others).

Holding that off until fewer changes are in the pipeline -- this is one of those merge-conflict-with-any-other-derive things.

[chrysn]

Un-holding it: the only thing this'd conflict with is the defmt experiments (#369), and they'll need a do-over anyway.