Support `FnOnce`, `FnMut`, `Fn` traits

[CohenArthur]

This issue serves as a tracking issue to collect test cases and interesting behaviors. It is far from exhaustive

• Handle passing functions as argument by reference #1481
• Handle passing functions as argument by value #1482
• Handle passing functions as argument by value through generic types #1483
• Error out when explicitly specifiying the desugared type of Fn traits #1484
• Forbid storing an impl Trait inside types #1485
• Allow passing an impl Trait as argument #1487
• Allow returning an impl Trait from functions #1486
• Figure out the restrictions surrounding impl Trait uses
• Handle storing references to functions in types #1488
• Handle storing functions by value when using generics #1489
• Handle storing multiple functions properly #1490

Any input is welcome! I'll open the above issues with corresponding test cases

[powerboat9]

I found a few related attribute issues with https://godbolt.org/z/9rPG9Wr1j

• Attribute rustc_on_unimplemented #2087
• Feature/Attribute fundamental #2088
• Attribute rustc_specialization_trait #2089
• Feature lang_items #2090