Allow typed functions in type expressions

[markwpearce]

• Allows syntax like:

sub useCallBack(callback as function(name as string, num as integer) as string)
    print "Result is: " + callback("hello", 7)
end sub

• Adds validation on typed functions used as arguments
• Provides better hovers for typed functions
• Fixes some issues with using Type statements
• Adds docs

[luis-j-soares]

Still trying to nail down how the entire type system works, but fwiw it looks good, and works without issue from the little I tested on my project.

A potential gap is recursive function types, which is not reflected in the tests. Also an outdated example in the docs.

[markwpearce]

Still trying to nail down how the entire type system works, but fwiw it looks good, and works without issue from the little I tested on my project.

A potential gap is recursive function types, which is not reflected in the tests. Also an outdated example in the docs.

Good point! I'll add a test to show this.

[TwitchBronBron]

This looks great! The tests really seem to cover all the bases. Fantastic work! Feel free to merge once you're done making changes.