String representation in FFI

[mversic]

#2625 makes an assumption that String can be transmuted into Vec<u8>. Usually repr(Rust) structures don't offer any guarantees on the representation of the object in memory. However, this comment made some points in favor of making this assumption.

We should consider special handling String conversion in FFI instead of transmuting it into Vec<u8>

[Erigara]

After some investigation it was found that repr(rust) single-field structs is guaranteed to have the same memory layout as it's single field (Unsafe Code Guidelines Reference).

Currently String is implemented as single-field struct (source) so it's transmutable into Vec<u8>.
There is still chance that String layout will be changed in the future, but this seems unlikely and could be fixed later.