Port to jbuilder and remove depopts

Rearrange the opam packages to eliminate optional dependencies,
and have explicit and separate packages for the Lwt, Async and
Mirage implementations. The opam and ocamlfind layouts now
have the same names:

• dns: the core package
• dns-lwt: Lwt implementation
• dns-lwt-unix: Lwt Unix, including servers
• dns-async: Async implementation (this currently uses Unix)
• mirage-dns

This layout is not compatible with the older releases which had
ocamlfind subpackages, so an upper bound will be needed in OPAM
for those. However, porting should be relatively straightforward
to the new release, and in return your users will not have to deal
with specifying a myriad of optional dependencies in OPAM.

• This release also ports the build to use Jbuilder, which speeds
  it up quite a bit and removes boilerplate files.

• Depend on Lwt 3.0.0 interfaces, including the blocking bind.

0.20.1: Merge pull request #141 from samoht/doc

0.20.1 (2017-05-16)

• Port to lwt >= 3.0 (#136, @djs55)

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.

0.19.1

0.19.1 (2017-02-15)

• Use topkg instead of oasis (#126)
• Do not reverse the order of resource records in the parser (#109)
• Restrict to OCaml 4.03.0+.
• Fix bug parsing pointers to pointers to DNS name labels (#129)

DNS parser fixes

• Use topkg instead of oasis (#126)
• Do not reverse the order of resource records in the parser (#109)
• Restrict to OCaml 4.03.0+.
• Fix bug parsing pointers to pointers to DNS name labels (#129)

MirageOS 3 + async maintenance

• Port to MirageOS 3 module types.
• Remove runtime dependency on PPX from META file
• Bugfixes and improvements for async backend compilation (#100 by vbmithr)

Clear AA bit on requests

• Do not set the AA bit on queries, as some servers will drop them

Uses ppx rather than camlp4

• Remove dependency on camlp4, switch to ppx

Clarify licensing and bump minimum compiler to 4.02.3

• This library now depends on the hashcons package rather than
  containing a fork of it. Now that there is no LGPL (+ linking exception)
  code left, clarify that the license is ISC. Previously the opam
  file claimed a mixture of licenses (#86 via @djs55).
• Add multi-distro Travis testing support.
• Library now depends on OCaml 4.02+

Multicast DNS improvements and port selection stability

• Change source port randomization to avoid overflow in the port range
  (#83 from @yomimono).

Improve mDNS support (#82 from Luke Dunstan):

• Add Dns.Probe to implement the unique name probing portion of mDNS.
• Expose the Dns.Name.Set construct.
• Added a functor Mdns_resolver_mirage.Chain that is intended to compose
  an mDNS resolver with a normal DNS resolver such that *.local is resolved
  via mDNS and everything else is done with DNS.
• Changed Dns.Query to not respond to queries for classes other than IN.
• Fixed mDNS legacy responses to use TTL <= 10 sec
• Fixed mDNS responses to use RD=0.