message: [lang] Wrap msg gen in mod to not pollute namespace
diff: @@ -283,14 +283,14 @@ fn codegen_for_instantiate(tokens: &mut TokenStream2, contract: &hir::Contract)
  let msg_toks = if message.is_mut() {
  quote! {
  - .on_msg_mut::< #camelcase_msg_ident >(|env, #msg_fn_args_toks| {
  + .on_msg_mut::< msg::#camelcase_msg_ident >(|env, #msg_fn_args_toks| {
  let (handler, state) = env.split_mut();
  state. #msg_ident (handler, #msg_call_args)
  })
  }
  } else {
  quote! {
  - .on_msg::< #camelcase_msg_ident >(|env, #msg_fn_args_toks| {
  + .on_msg::< msg::#camelcase_msg_ident >(|env, #msg_fn_args_toks| {
  let (handler, state) = env.split();
  state. #msg_ident (handler, #msg_call_args)
  })
  @@ -491,11 +491,14 @@ fn codegen_for_messages(tokens: &mut TokenStream2, contract: &hir::Contract) {
  content
  };
  tokens.extend(quote! {
  + mod msg {
  + use super::*;
  // Apparently this `use` is required even though it should not be.
  // -> Further investigations needed!
  use ink_model::messages;
  ink_model::messages! {
  #messages_content
  }
  + }
  })
  }
  