Change Hir Creation API

[lausek]

// old
hir.step(Assign::var(n, 2));
// new
hir.assign(n, 2);

• Move all HirElement constructors as methods on HasBlock
  • Assign: assign, increment, decrement
  • Repeat: break_repeat, continue_repeat
  • Import: import, import_from
  • Interrupt: trigger
  • Return: return_nil, return_value
• Improve Expr builder
  • Conv: to_bool, to_float, to_integer, to_str
  • Iter: to_iter, create_ranged, has_next, next, reverse