Remove Dns.Buf and make allocation simpler

• Remove the Dns.Buf module that formerly wrapped Cstruct, now that the
  latter is a mature library.
• Add an ?alloc optional argument to functions that would formerly accept
  a Dns.Buf. By default, this allocates a single page, but consumers of
  this library can override it in order to supply their own allocation
  logic (e.g. a pool allocator).
• Remove dependency on io-page to follow the Dns.Buf removal.

All these changes were part of #132 by @hannesm.