[Enhancement] Support isFunction<T>()

[jtenner]

I'm developing an assertion library, and it would be helpful to be able to assert if a type is a function.

A simple use case is the toThrow() expectation function.

  @inline
  public toThrow(message: string = ""): void {
    // assert(isFunction<T>(), "toThrow expectation must be called on a function type.");
    var throws: bool = !tryCall(this.actual!);
    reportActualString<string>(throws ? "throws" : "not throws");
    reportExpectedString("throws", this._not);
    assert(this._not ^ throws, message);
    clearExpected();
  }