Make augmented assignments and assoc return their targets

[refi64]

Ref #233, closes #238.

[gilch]

This breaks augmented slice assignments.
Current:

=> (setv spam (* 10 [0]))
spam = (10 * [0])
=> spam
spam
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
=> (print (+= (slice spam 2 5)[1 2 3]))
spam[2:5] += [1, 2, 3]
print(None)
None
=> spam
spam
[0, 0, 0, 0, 0, 1, 2, 3, 0, 0, 0, 0, 0]

Proposed:

=> (setv spam (* 10 [0]))
spam = (10 * [0])
=> spam
spam
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
=> (print (+= (slice spam 2 5)[1 2 3]))
spam[2:5] += [1, 2, 3]
print(_hy_anon_var_1)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
NameError: name '_hy_anon_var_1' is not defined
=> spam
spam
[0, 0, 0, 0, 0, 1, 2, 3, 0, 0, 0, 0, 0]

Similarly breaks augmented subscript assignments:

=> (print (+= (get spam 0) 12))
spam[0] += 12
print(_hy_anon_var_1)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
NameError: name '_hy_anon_var_1' is not defined

And augmented attribute assignments:

=> (setv foo (lambda [] nil))
foo = (lambda : None)
=> (setv foo.bar 2)
foo.bar = 2
foo.bar
2
=> (print (+= foo.bar 1))
foo.bar += 1
print(_hy_anon_var_1)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
NameError: name '_hy_anon_var_1' is not defined

[paultag]

@gilch @kirbyfan64 what's the status of this? Can we update against master and address feedback?

Please re-open once it's updated!