Add support for removing shapes from WF specifications

[EliasC]

This PR adds support for removing shapes from a WF specification:

inline const auto wf1 =
  (Foo <<= Bar++)
| Baz <<= Foo * Bar
;

inline const auto wf2 =
  (wf1 - Foo)
  | Bar <<= Baz++

In wf2 Foos can still appear in the tree, but they must have zero children (just like if they were never in the WF to begin with). Without this PR, the only way to achieve the same functionality is by repeating the shape list without Foo.

To be honest, I'm not sure what I'm doing with the functions taking rvalues, I'm just parroting other similar functions.

[mjp41]

LGTM