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.

Initially I meant to ensure the allocator gave back sequential memory in
segment0, but ended up using a hope+validate approach. Given this
approach, modification of this function adds useless complexity.

kj/vector: add methods operator = nullptr and truncate

Previously I assumed that data sections in the input StructReader must
have data sections which were multiples of words. However, struct
upgrade rules allow a StructReader to exist which does not follow that
rule. This commit removes that assumption by working in bytes normally,
and special casing the 1-bit-struct case.

=nullptr releases memory

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

The user facing API is in MessageReader and MessageBuilder

{MessageBuilder,MessageReader}::isCanonical verifies the canonicity of a
message. This is both useful for debugging and for knowing if a received
message can be used for hashes, bytewise equality, etc.

MessageBuilder::canonicalRoot(Reader) can be used to write a canonical
message on a best effort basis, and checks itself using isCanonical.
It should succeed as long as the MessageBuilder in question:
* Has a first segment which is long enough to contain the message
* Has not been used before

Tests have been added in canonicalize-test.c++ which verify that for
crafted examples of canonicalization errors, isCanonical will reject,
and for a canonicalized version of the standard test message, it will
accept.

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