General cleanup

- Make some errors nicer.
- Remove dead code. (We're depending on hg tip anyway, so it's okay to
  use its features.)
- Correct the documentation for (*Conn).Send().
Ignore unexported struct fields in Store

Also, document that unexported fields are ignored for encoding and
decoding.
Handle org.freedesktop.DBus.Peer on any path

Also, document this.
Handle method calls that lack an interface

The specification states that if the interface header is missing for a
method call, but the method name is unique, the method must be called.
If the method name is not unique, the behaviour is undefined. We take
the easy way out and always call the method on the first interface that
is found when iterating over the map.
Speed up alignment lookup

benchmark                         old ns/op    new ns/op    delta
BenchmarkDecodeMessageSmall           59660        37788  -36.66%
BenchmarkDecodeMessageBig            114932        72561  -36.87%
BenchmarkEncodeMessageSmall           48470        25457  -47.48%
BenchmarkEncodeMessageBig             85963        43824  -49.02%

Thanks, pprof.