lobstr 1.2.0

• New src() function for exploring srcref and srcfile objects. We've
  documented all we know about srcrefs in ?src.

• obj_size(), obj_addrs(), and sxp() no longer error with "bad binding
  access" when inspecting environments with non-standard bindings such as
  those created by for loops or immediate bindings (#48).

• sxp(expand = "environment") no longer shows the internal _frame and
  _hashtab structures. Instead, it now shows promise expressions without
  forcing them. This change was necessary to make lobstr compliant with R's
  public C API.

• General progress towards conformance to the public C API of R.