Closes #408. Perhaps the same bug as in commit cff9ba05?

Example:

    const data :Data = embed "some-file.dat";

Files are looked up the same way an import would be.

You can use embed when Data or Text is expected. You can also use it when a struct type is expected -- the file will be interpreted as a message using standard binary serialization.

Add asBytes() and asChars() methods to array classes to reinterpret-cast to bytes / chars, since this happens all the time and is otherwise a huge pain.

Use the new methods in a bunch of places.

Thanks to Bryan Boreham <bjboreham@gmail.com> for much help getting this started.

This prevents the compiler from reporting warnings in these headers while compiling application code. Hopefully this will stem the never-ending stream of complaints from people who enable pedantic warnings.

More refactoring is needed, but this at least makes "using" aliases work correctly with generics.

For portions currently copyright by Kenton (most of it), transfer copyright to Sandstorm Development Group, Inc. (Kenton's company).

The license change is practically meaningless, as MIT and BSD 2-clause are legally equivalent. However, the BSD 2-clause license is sometimes confused for its ugly siblings, BSD 3-clause and BSD 4-clause. The MIT license is more immediately recognizeable for what it is.

Rémy Blank and Jason Choy (the two non-trivial contributors) are on record as approving this change:

https://groups.google.com/d/msg/capnproto/xXDd2HUOCcc/gbe_COIuXKYJ

Pull TextBlob out of capnpc-capnp and into KJ as kj::StringTree.  Use it in the stringification code to make that code cleaner and avoid using iostreams.

Add utility code for really nice command-line option parsing and start using it in the compiler (still WIP).

Implement mutex and once-init synchronization primitives.  Wrap pthreads for now, but plan on raw futexes later.

Enable some useful Clang warnings, fix some bugs it detected.  Especially, eliminate all dynamic initializers.

Mark every destructor "noexcept(false)" unless it is very clearly noexcept.  The change in C++11 making destructors default-noexcept was a mistake.