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.

I'm not really sure how Cap'n Proto has worked at all on GCC 5. For some reason this only ended up being tickled after e6e29122.

Fixes #442.

Fixes https://github.com/sandstorm-io/ekam/issues/13
Fixes https://github.com/sandstorm-io/ekam/issues/11

Fix build with -fno-exceptions

This includes making builders a little more tolerant of corrupt data. Note that our threat model generally does not expect this tolerance -- we expect that builders always contain either structures created locally or copied in from a reader, which does a certain amount of validation in itself.

Actually implement integer overflow guards