Apply some capnproto -> capnp renames that didn't happen before, update docs, put samples in a place where they are easy to compile.

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

update automake include directory to capnp

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

Add mechanism for catching exceptions that has limited use even with -fno-exceptions, as well as a mechanism to detect when a destructor is called during unwind and to avoid throwing in these situations.

Use consistent naming style for structs that exist purely to specialize template aliases or functions.

Improve const-correctness of readers and builders.  We enforce transitive constness, meaning you should not be able to construct a mutable Builder from a const Builder (you can always convert them to Readers instead).