Reduce amount of generated code when deriving Configs in Scala 3

[kyri-petrou]

While working on a previous issue, I realised that the derivation of DeriveConfig was generating a lot of unnecessary bytecode in Scala 3. This PR reduces it substantially by:

1. Using val instead of lazy val within inlined methods
2. Putting transformations within non-inlined methods to avoid generating the same FunctionX whenever we derive a new DeriveConfig
3. Using @threadUnsafe whenever we need to use a lazy val but don't plan to access it concurrently by multiple threads