This change takes a comment about lazy execution from
https://github.com/sandstorm-io/capnproto/issues/263#issuecomment-158564701 and
adds it to the C++ RPC documentation.

Although lazy execution is described in `Promise::then()` method documentation,
it's unfortunately buried in the *Advanced implementation tips* section, and
described as an optimization rather than as a more general feature of the
asynchronous execution model. Since I recently got tripped up by this behavior,
I thought it would be worth trying to document better.

omit needless memset

remove unneeded kj-async dependency in Makefile.am and capnp-json.pc.in

fix various bugs in canonicalization

fix typo in canonicalization docs

fix bug where PointerBuilder::getPointerType() can corrupt PointerBuilder::segment

update mention of non-existent 'stepBits' field

Make sure to clear tag word when upgrading a struct list.

Edit error message for the case where copyPointer() hits a FAR pointer.

The existing error message suggests this case can only arise if there is a
a bug in the library. However, malformed input can trigger the error too.
In particular, the error gets thrown when a non-double far pointer resolves
to another far pointer.

fix bug in parsing of non-ascii constants

Fix MSVC build errors

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

Add HTTP client and server implementation.

The old semantics were intended to allow more implementation freedom, but I find in my actual code it's very convenient to rely on the actual implementation details. Other implementations will just have to deal with it.