[Divyansh089]

implements defmt::Format for every public type that implements Debug:
Drain, IntoIter, ExtractIf, Splice and SmallVecError (SmallVec already had it).

• all impls are gated behind the defmt feature.
• each one prints the same as its Debug impl.
• SmallVecError is implemented by hand, since core::alloc::Layout doesn't implement Format.

added tests/defmt.rs, which checks at compile time that each type implements Format.

Closes #795

[alejandro-vaz]

implement it for RawSmallVec and LocatedLength as well, along with Debug

[Divyansh089]

done.

• added Debug and defmt::Format to both
• locatedLength prints its length and on_heap. RawSmallVec prints RawSmallVec { .. }: the union doesn't know which member is active (locatedLength tracks that), so reading either one here would be UB

[alejandro-vaz]

looks good. thanks @Divyansh089