fix(common): Use zero-copy deserialization when FromStr is implemented

[jan-auer]

We sometimes implement both FromStr and serde::Deserialize. In cases where
we share the implementation, we provide an explicit definition of FromStr and
use macros to implement serde::Deserialize.

There were two implementations of this macro:

1. relay_common::impl_str_serialization
2. relay_general::impl_str_serde

The implementation in relay_general allows to implement just deserialization
or serialization, but had a bug: It is not zero-copy. Instead, it always
allocates a String, only to then parse it.

In this PR, we deduplicate the macro and ensure that there are no allocations
when deserializing.

#skip-changelog