doc/cxxrpc: use getParams() in server

Server::Context does not have method getRequest

Add missing isRunnable implementation

*.pc.in: add exec_prefix lines

The removal of exec_prefix in a900f200
caused the generated .pc files to be unusable when built with autotools.

The generated libdir value would read:

  libdir=${exec_prefix}/lib

but since exec_prefix was unspecified, pkg-config would choke:

  zarvox@localhost $ pkg-config capnp-rpc --cflags --libs
  Variable 'exec_prefix' not defined in '/usr/local/lib/pkgconfig/capnp-rpc.pc'

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Fix some outdated comments and error messages.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.