Remove key args and both

[samoht]

To test with: mirage/mirage-skeleton#364

Based on top of #1437 - experiment with removing "both" keys (just keep runtime and configure-time keys). Also remove the ~key parameter when registering unikernel to force people to move these into the unikenerls.

Should address most of what was requested in #1422 - but it's a bit more radical than #1437 so opening a separate PR to discuss.

[hannesm]

To me, this looks like the right direction on how to make unikernel development easier to understand -- the keys are now defined in unikernel.ml, and can be accessed there directly. Should the return value from register (of a key) be a lazy 'a instead of unit -> 'a?

This also allows in unikernels that use other types than the base ones to specify converters for them (i.e. Ipaddr / Domain_name / X509.Key_type.t / ...). That's just great, I've been looking at that for a decade ;)

IIUC, Key_gen is still used, mainly for these keys for the network stack etc. (which are barely used in the unikernel, but the network stack uses it internally to configure itself).

Did you try to port a more complex unikernel, such as mirage-www or unipi, to this branch? That'd likely be a good test on how this pans out.

[samoht]

This has been merged as #1451, #1450, #1449, #1448, #1447 and #1437 (the main one).