remote: add AddToStoreNar operation

[squalus]

Some open items:

• What type should be used for the reply? Nothing is sent over the wire. () is already taken by opSuccess which reads a boolean. Maybe opSuccess should use a new reply type, and this one should use ()
• The client chooses the chunk size. What should that be?
• Better errors for the RemoteStoreError_Fixme usages
• Make an Arbitrary instance for SampleNar

Closes #265

[sorki]

Very nice (especially the framing!) and sorry for the conflicts! Wanted to merge earlier but didn't get to it sooner.

The CheckSignatures is also in core (I've realized it is actually CheckSignatures as well only couple days ago) - but don't worry about it, I'll deal with it.

You're right about opSuccess, most ops simply return True and indicate errors via logger so it seemed appropriate to just use () but this one makes it weird, I agree about the newtype.

No idea about the chunk size tbh.

[squalus]

• Replaced the existing () reply type with a new SuccessCodeReply type
• Added a NoReply type for this case
• Added error types
• Resolved conflicts

[sorki]

Nicely done, thank you!

[sorki]

Btw I'm pretty much done with most large-scale changes/refactors so things should settle down now, feel free to tackle what you see fit.