Support introduction patterns /=, {H}, {$H} in iDestruct.

This fixes issue #57.

I considered supporting these introduction patterns also in a nested fashion --
for example allowing `iDestruct foo as [H1 [{H1} H1 /= H2|H2]` -- but that
turned out to be quite difficult.

Where should we allow `/=`, `{H}` and `{$H}` exactly. Clearly something like
`>/=` makes no sense, unless we adopt to some kind of 'stack like' semantics
for introduction patterns as in ssreflect. Alternatively, we could only allow
these patterns in the branches of the destructing introduction pattern
`[... | ... | ...]` but that brings other complications, e.g.:

- What to do with `(H1 & /= & H3)`?
- How to distinguish the introduction patterns `[H _]` and `[_ H]` for
  destructing a spatial conjunction? We cannot simply match on the shape of the
  introduction pattern anymore, because one could also write `[_ H /=]`.