Make sure valuess are used/copied before they are freed

While reviewing  src/parser.y, I noticed that in a bunch of places
the code relied on arguments being evaluated in a certain order.

This seemingly is not currently causing problems, but with

  func(foo(jv_copy($1)), bar($1));
  func(foo(jv_string_value($1)), bar($1));

It is not guaranteed that jv_copy()/jv_string_value() is evaluated
before bar().
This patch splits those function calls into multiple statements to
ensures values are used/copied before they are freed where necessary.