This PR refactors the compiler sources with the intent to eliminate any redundant type assertions that might involve a redundant dynamic check at runtime when compiling to WebAssembly.

Typical patterns are

if (element.kind == ElementKind.FUNCTION) {
  doSomething(<Function>element);
  doSomethingElse(<Function>element);
}

which are nops in TS but upcasts in AS, where refactoring these to

if (element.kind == ElementKind.FUNCTION) {
  let functionInstance = <Function>element;
  doSomething(functionInstance);
  doSomethingElse(functionInstance);
}

does just one dynamic check instead of two at the expense of an additional local that Binaryen will most likely collapse again.

Also makes relevant code a little more verbose as a side-effect, which some might find helpful to keep track of things when working with the sources.