Support postfix expressions as function identifiers

[hadronized]

The following code will trigger a parse error on the current release / HEAD:

vec3[3] verts = vec3[]( /* whatever */ );

Here, the vec3[](…) is a function call, and the function identifier must be vec3[]. Currently, it’ll fail to parse.

This is not easy to fix because the function call parser is already defined in the postfix expression parser as an alternative. We have to be smart to fix that.