fix: allow call expressions in assignment

[brandonspark]

Currently, we cannot parse programs which have things like:

Foo(context).elem = var1

which assigns var1 to the property elem of the object returned by Foo(context). You can see in the Kotlin playground that such code parses, and is attempted to evaluate:
https://pl.kotl.in/E1IajnqkH

This PR just moves call_expression so it may appear in an assignment LHS.

[brandonspark]

@fwcd

[brandonspark]

@fwcd do you think youll have any time to look at this soon? it's moderately blocking some parsing problems for us. thank you!

[fwcd]

Thanks!