S()-based scope assignment

[mahmoud]

Per discussion, here's a sketch of what it looks like to assign to the scope using S(var=spec). Personally I like the brevity and symmetry, and not needing another import.

Topics for further discussion:

• [x] Prior to this PR, we allow S(), even though it's always scope and will always fail at runtime. A small bug.
• [x] ~Should we accept a dict posarg for non-kwarg update?~
• [x] ~Should we typecheck kwarg values?~
• [x] Is there a better way of doing this than if cur is scope?

[codecov[bot]]

Codecov Report

Merging #174 into master will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #174   +/-   ##
=======================================
  Coverage   98.87%   98.88%           
=======================================
  Files          26       26           
  Lines        3998     4029   +31     
  Branches      567      575    +8     
=======================================
+ Hits         3953     3984   +31     
  Misses         21       21           
  Partials       24       24           

Impacted Files	Coverage Δ
glom/__init__.py	100.00% <ø> (ø)
glom/core.py	98.65% <100.00%> (+0.01%)	:arrow_up:
glom/test/test_basic.py	100.00% <100.00%> (ø)
glom/test/test_match.py	100.00% <100.00%> (ø)
glom/test/test_mutation.py	97.94% <100.00%> (ø)
glom/test/test_scope_vars.py	100.00% <100.00%> (ø)

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[mahmoud]

Items discussed are done. There's the small issue of this being documented, but I'm willing to defer documenting to gather usage feel and let it bake. And of course expedite v20.7.0

[mahmoud]

Talked about it, will make the docs changes.