Compound assignment

[Mercerenies]

Consider allowing operators such as +=. So the following desugaring process would occur

;; Original statement
foo bar += 1.
;; Desugar the compound operator
foo bar = foo bar + 1.
;; Desugar the accessor
foo bar= (foo bar + 1).

With the exception that foo would only be evaluated once. This would, strictly speaking, be a breaking change, as foo bar += 1 is currently a valid statement in Latitude.

Additionally, we would need to consider what operators this should work for. It can't work for things like == or ===, or a rather silly desugaring process would take place.

a == b.
a = a = b.
a= (a = b).

Which is obviously undesired, not to mention syntactically invalid.