Generate separate .pc files for kj and kj-async

Include kj.pc, kj-async.pc in automake build

Add function to predict the size of a message from its prefix (and detect if the message is complete).

This is largely motivated by the fact that Clang currently miscompiles `.then([](auto){})`:

https://llvm.org/bugs/show_bug.cgi?id=24989

Fix link order in FindCapnProto.cmake

As of that change, adopting an empty struct into a pointer that already had some other value fails to overwrite the upper 32 bits of the pointer, thus resulting in a struct pointer with totally bogus bounds. Normally this will either cause a bounds check exception to be thrown when that pointer is later accessed, or lead to a struct containing bogus data (but if teh struct was expected to be empty, probably that data is never accessed).

json: Hide the input array from parser methods

Fix some outdated comments and error messages.

Add missing case analysis in handleUnimplemented().

Support extracting the current exception count on MSVC2015.

The MSVC runtime was heavily refactored, so the internal function we call to
get the current exception count changed for newer MSVC versions (as did the
offset).

Account for the possibility of adopting an empty struct.

This encapsulates the input array, hiding it from the parser's methods.
Access is forced to go through `advance` and `consume*` methods on the
new `Input` object. The aim is to make it harder to have errors like
https://github.com/sandstorm-io/capnproto/pull/270

adjust the set of white space characters to the set described by RFC7159

Check num remaining bytes of escaped json character

Prefer display:block over inserting new <p> elements.

Drop onlick() handlers in documentation, in favor of simple links.