style-guide: fix isConst and UnConst examples

Problems:
* missing ; after struct declarations
* missing static for constexpr bool value
* UnConst_<T> -> UnConst_<const T>
* UnConst = -> UnConst = typename

json: integrate to build

In Sandstorm, we are encoding powerbox queries in packed base64 strings which may be placed in URL query parameters or the like. The strings are provided in interfaces called browser-side. We anticipate that some developers will prefer to specify a hardcoded string rather than generate it on-the-fly, since Cap'n Proto is not well-supported in browsers today, and anyway the developer may have no other reason to have a Cap'n Proto dependency at all, and powerbox queries are often static.

In this context, speed is irrelevant, while having a compact encoding is desirable. It felt sad to me to leave in the segment table in this context, adding redundant bytes when we want a compact encoding.

Fix uninitialized members of ListBuilder

Adding a `KJ_DASSERT` in the `setListPointer` logic flagged
non-word-multiple data sections in `INLINE_COMPOSITE` lists, which
should be impossible. This traced back to uninitialized member variables
in `ListBuilder` in the case that it was created from a null pointer.

kj/vector: add methods operator = nullptr and truncate

=nullptr releases memory

https://github.com/sandstorm-io/capnproto/issues/292

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.