Use absolute import path for c++.capnp.

This allows {schema,rpc,rpc-twoparty}.capnp to be used without modification when
relocated outside of the Cap'n Proto codebase.
Explicitly mark IndexingIterator as a RandomAccessIterator
Merge pull request #86 from kevinwallace/schema-absolute-import

Use absolute import path for c++.capnp
Provide a specialisation of std::iterator_traits for IndexingIterator.

Only available when KJ_STD_COMPAT is defined.
Merge pull request #88 from partylemon/Iterator

Explicitly mark IndexingIterator as a RandomAccessIterator