QOlmExpected and associated refactoring

As mentioned in the commit introducing `Expected`, `QOlmExpected` is
simply an alias for `Expected<T, QOlmError>`. This simplifies quite
a few function signatures in `QOlm*` classes and collapses unwieldy
`std::holds_alternative<>`/`std::get<>` constructs into a neat
contextual bool cast and an invocation of `operator*` or
`value()`/`error()` accessors that don't need to specify the type.

While refactoring the code, I found a couple of cases of mismatching
`uint32_t` and `qint32_t` in return values; a couple of cases where
`decrypt()` returns `QString` which is in fact `QByteArray` (e.g., in
`QOlmSession::decrypt()`); there's a repetitive algorithm in
`Connection::Private::sessionDecryptPrekey()` and
`sessionDecryptGeneral()`

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