codegen: emit has_foo accessors for pointer fields
tests: generate code for 'test.capnp' from capnproto repo
tests: early work on encode/decode tests
tests: two's complement math
runtime: use int64 literals for pointer bitmasks
runtime: fixes for composite list bookkeeping
tests: provide floating-point equality test for encode/decode
tests: functorize decode test so it works with reader and builder