Described Mixin

[phadej]

Related to #6281

(black dots are where space is required).

[phadej]

I think

mixins: pkg(Module)requires(Module2)

is accepted by the parser but not precisely Described. The whitespace is tricky when we have infix keywords and empty productions. E.g.

mixins: pkg requires(Module2 as Module3)
mixins: pkg requires hiding{Module4}