replace left stream on lhs of assign

Replace the streaming operator on the left-hand side of an assignment
statement with a series of assignment statements, one for each slice.

e.g.:

  logic [3:0] a, b;

  {<<{a}} = b;

becomes:
  a[3] = b[0];
  a[2] = b[1];
  a[1] = b[2];
  a[0] = b[3];
add right streaming operator support
avoid creating new AstAssign node
regression test for stream operator: << and >>