Add an easier way to read a message directly into a builder. Requires a copy for security/validation.

This is not currently used, but eventually we'll use this to support efficient inter-thread messaging.

That said, I've decided that inter-thread messaging is complicated and I don't want to work on it right now.

CMake support for MSVC/Visual Studio

fix some typos

Wait for writeable, rather than readable, socket on connect.

CMake automatically patches the switch character.

CMake: Fix #149.

Check for the definition of `_MSC_VER`, rather than its value.

Conflicts:
	c++/src/kj/common.h

Distinguishing between "local bugs" and "preconditions" was proving difficult in practice, because a precondition failure in one function may very well indicate a bug in a calling function, but the exception may be thrown through that function, thus when caught the classification is nonsensical. The distinction also was not as useful as imagined. So, I eliminated this distinction.

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.