Improve proc-macro rust user experience

[the-orex]

1. Make top instruction wrapper non-optionnal (by implementing ::prost::Message manually when proto feature is activated)

To change consuming from:

   let Some(ref ix_inner) = ins.instruction else {
         return Ok(());
   };
   
   match ix_inner {

into

match &ins.instruction {

2. Change struct variant for Instruction enum (by implementing ::prost::Oneof manually when proto feature is activated)

To change consuming from:

Ins2SpanningParserOutput::InsSub(ix) => {
                match &ix.instruction {
                    cpmm_instruction::Instruction::Initialize(inner) => {
                        let accounts = &inner.accounts;

into:

Ins2SpanningParserOutput::InsSub(ix) => {
                match &ix.instruction {
                    cpmm_instruction::Instruction::Initialize { ref accounts, .. } => {