bug(generator): cannot generate when go 1.18+ generics are used.

[itsdevbear]

Generator blows up really hard on generics, would be nice to handle.

[letonchanh]

Currently, decodeASTStruct does not consider TypeParams of ast.TypeSpec. That's why structs with generics cannot be generated. To support generics, I think fastssz also needs to decode TypeParams.

[ferranbt]

Do you have an elaborated set of examples on how do you want to use generics? I am trying to understand the full scope. The example in #160 should be fairly easy to handle in itself.

[ferranbt]

This is a fix to encode the struct in #160.