generate should return string rather than Cstuct.t

[oemmerson]

Previously building these examples would fail with the error

Error: This expression has type int -> Cstruct.t
       but an expression was expected of type int -> string
       Type Cstruct.t is not compatible with type string
make[1]: *** [build] Error 1

[reynir]

Thank you for your PR! The recent randomconv.0.2.0 release changed the expected type from Cstruct.t to string in anticipation of an upcoming mirage-crypto release that brings a cstruct->string overhaul.

I would say the preferred way forward is to set an upper bound on randomconv<0.2.0 for now.

[oemmerson]

@reynir Thanks for explaining. I've reverted the original commit, added the upper bound you suggested and checked the kernel's build without issue, so you can squash them or just cherry pick the final commit if you want the changes.

[reynir]

Thanks a lot!