`#generate_revCDeriv` generates incorrect name

[lecopivo]

Calling #generate_revCDeriv with trailing arguments produces an incorrect name

#generate_revCDeriv matmul A | i 
  prop_by unfold matmul; fprop
  trans_by unfold matmul; autodiff; autodiff

generates function matmul.arg_A.revCDeriv but it should generate function with the name matmul.arg_A_i.revCDeriv

see test/generate_ftrans.lean for (non)working example