Unsafe enum usage

[Koka]

Passing mut enum across FFI boundaries causes UB in cases when ODBC driver returns value absent in enum. Details: https://users.rust-lang.org/t/undefined-behaviour-after-unsafe-enum-usage/15572

[Koka]

NB: As a part of this fix we need to update odbc-sys to v 0.6.0 at least

[Koka]

There is rust-lang/rust#44109 which is not implemented yet, but seems to be able to fix this problem

[scottmcm]

Note that [non_exhaustive] doesn't solve the unknown-discriminant-in-FFI case -- the crate that defines the enum is still allowed to exhaustively match on all the variants that exist (because it's the crate that owns the enum, so can update the match along with the new variant, if one is added) and thus it's still UB to put an unknown discriminant in an enum.