Alternate cfg pattern

[alephzero]

Alternate cfg implementation with much less magic.

Previously, cfg was an object wrapper that would masquerade as the requested type in almost every way. The two major exceptions was 1) type(obj) would not match the underlying type, and 2) C-bindings were not fooled.
The masquerading objects were hooked and updates when the user calls a0.update_configs()

New cfg2 has the same signature, but returns a non-masquerading object that has an explicit get() method. get() returns a true object of the requested type, not one that updates under the hood.
get() always returns the most recent version. No need for an update call.

Example usage here:

bar = a0.cfg2("topic", "/bar", int)
assert bar.get() == 3

Ideal transition plan:

• This new pattern is called cfg2
• Once this lands, we can update users from cfg to cfg2
• When no users of the old cfg exists, alias cfg==cfg2
• Update users from cfg2 to cfg
• Remove cfg2
  We'll see how well that turns out.