Stop generating dart functions

[cgrand]

Currently clojure functions compile to either classes or functions.

Here I propose we get rid of emitting Dart functions and always compile to classes because:

• Dart overcaptures Dart VM closures are not safe-for-space (share the same context if defined within the same scope - leading to overcapturing) dart-lang/sdk#36983
• it would simplify the compiler, the generated code (we wouldn't need to consider Function on the fast path -- Functions would remain callable) and the mental mode

However the compiler would keep the special case that when it statically determines something in function position to be a Function, it will invoke it with no overhead.

We should double-down on magicasting to Function (we need to anyway)

[cgrand]

The doc for build on State also warns against accidental this capturing.