Implemented as<T>() and is<T>() with accompanying tests

[JulianvDoorn]

Solves issue #1066

[dota17]

Run ninja -v -C build-${LIB_TYPE} clang-format before submitting your code to format your code style, otherwise, it will fail in Travis-CI. And of course, you should run ninja -v -C build-${LIB_TYPE} to build your project to see if there is any build error after your changing. please see CONTRIBUTING.md

[BillyDonahue]

We can avoid macros here. There are only a few types.
The "helper" structs are not needed.
They should have better names if they remain, and not be public.

We should be able to implement the as<T> and is<T> as member function template specializations directly, without forwarding to a helper struct.

[BillyDonahue]

Here you've invented a new accessor: is<const char*>.
It does the same thing as is<String> but has a distinct name. I think that's confusing and we should remove the const char* specialization.

[BillyDonahue]

This was finished off and pushed as PR #1080 . Thanks!