Allow nested invocation for short syntax

[Emoun]

Now that #1 has changed the short syntax, it should be possible to allow nested invocation in it:

#[duplicate(
  int_type  implementation;
  [i8]      [ *self < 0 ];
  #[
    int_type  ; [u8]; [u16]; [u32]
  ][ 
      [ int_type  ] [ false ]; 
  ]  
)]

[Emoun]

Initially, nested invocations cannot come before the list of substitution identifiers. This means, we can only really use it for producing substitution groups.

This could be changed in the future if the functionality is deemed worth it. Currently I cannot think of a good use case to being able to produce the identifier list through a nested invocation.