Or... negato?

Refactor bounds checks to avoid ever creating out-of-bounds pointer values, which is technically UB even if not dereferenced.

Details: https://github.com/sandstorm-io/capnproto/blob/master/security-advisories/2017-04-17-0-apple-32bit-elides-bounds-check.md

MallocMessageBuilder: don't allocate segments above the max serializable size

Fix autotools build on MacOS

This eliminates a TODO(soon).

This allows functions which use KJ_EXPECT and other test macros to be built without linking libkj-test, which pulls in a possibly-unwanted main() function.

In particular, afl-testcase.c++ could not link in -fno-exceptions mode without this change.

doc/cxxrpc: Document lazy execution

The previous approach worked only for ekam builds, where fuzz-test was its own binary.

Add AFL test case

I changed it to use TestAllTypes instead of a trivial struct, which will probably get more coverage quicker.