Generate haddock comments in compiler plugin output.

[zenhack]

It would be nice if the schema compiler plugin added haddock comments to the varioius declarations it generates; Most the time I end up referring to the schema files themselves for information, but my experience with the Go implemenation is that it's a small burden to have to do the mental re-mapping to actually generated functions/methods/type names, and it's either that or look at two sets of reference docs. It would be useful to be able to have generated haddock output that contains everything.

This will have to wait for the (I believe upcoming) 0.7 capnproto release, since the 0.6.1 schema compiler doesn't include the necessary information in its output. 0.7 includes it here:

https://github.com/capnproto/capnproto/blob/master/c%2B%2B/src/capnp/schema.capnp#L173

Once that's available, this should be really easy.

[zenhack]

0.7 is out now:

https://github.com/capnproto/capnproto/issues/694#issuecomment-416794008

...so this is unblocked. We'll need to update the Docker container we're using for CI to use 0.7, and use the newer version fo the schema in the code generator.