The whole IPLD APIs get a review to make them more consistent and
easier to use.

Changes to compared to the current spec:

 - No more callbacks, everything is Promises.
 - `get()`:
   - Is renamed to `resolve()` to indicate that it also takes a mandatory
     path argument.
   - Doesn’t have a `cid` in the return value anymore. That use case is
     covered by returning all objects that were traversed. Their `value`
     will always be the CID of the next one to traverse. So if you want
     to know the CID of the current IPLD Node, just look at the `value`
     of the previously returned IPLD Node.
   - Doesn’t return a single value, but an iterator.
   - `localResolve` option is removed. If you want to resolve a single IPLD
      Node only, just stop the iterator after the first item.
 - `getStream()` is removed without replacement. Use `resolve()` which uses
    async iterators instead.
 - `getMany()` is renamed to `get()`:
   - takes an iterable of CIDs as parameter.
 - `put()`:
   - takes an iterable of CIDs as parameter.
   - Doesn’t take `cid` as an option anymore. The CID is always calculated
     (#175).
   - The options don’t take the `format` (which is a string), but the `codec`
     (which is a `multicodec`) (#175).
   - the `version` option always defaults to `1`.
 - `.treeStream()` is removed without replacement (the only current user seems
   to be the IPFS DAG API tree command and the ipld-explorer-cli). IPLD Nodes
   are just normal JavaScript objects, so you can call `Object.keys()`
   recursively on a IPLD Node to get all its resolvable paths. If you want to
   follow all the CIDs, write the tree traversal yourself. This could perhaps
   be an example bundled with js-ipld.
 - `remove()`:
   - takes an iterable of CIDs as parameter.
 - `.support.add()` is renamed to `.addFormat()`.
 - `.support.rm()` is renamed to `.removeFormat()`.

Almost all open issues in regards to the IPLD API got adressed. One bigger
thing is the Patch API, which also isn’t part of the current specification.
Here’s an overview of the issues:

 - #66
   - [ ] IPLD Resolver `patch` API: There is no patch API yet
 - #70
   - [x] lazu value lookups: Can be done as IPLD Formats is pure JavaScript
   - [x] get external / local paths only: @vmx doesn’t know what this is,
         considers it a “won’t fix”
   - [x] toJSON + fromJSON - roundtrip: A roundtrip is not a goal anymore
         => won’t fix
   - [ ] put + patch api #66: Patch API is still missing
   - [x] text summary: @vmx doesn’t see a strong use case for this => “won’t fix”
   - [x] globbing: Out of scope for js-ipld
 - #175
   - [x] Deprecate passing a CID to `ipld.put`?: Current spec doesn’t allow
         `put()` with a CID
 - #182
   - [x] API Review: Links to many other issues, I list the individual issues
         below
 - #183
   - [x] getting the merkle proof with resolver.resolve: `resolve()` returns
         all CIDs along the traversed path
 - #184
   - [ ] Pass down the `options` object to resolver.resolve(): This needs a
         custom resolver. @vmx isn’t sure if the js-ipld API should make this
         possible, or of it should just be easy enough to create your own
         resolver.
 - https://github.com/ipfs/interface-ipfs-core/issues/81
   - [x] The `dag` API - One API to manipulate all the IPLD Format objects:
         Interesting discussion, but not relevant anymore.
 - ipfs-inactive/interface-js-ipfs-core#121
   - [x] dag api: add {ls, tree}: `tree()` is not part of js-ipld anymore as
         the IPLD Nodes are just JavaScript objects which you can easily traverse
         if you like. Though `tree()`-like functionality might be added as an
         example or separate module.
 - ipfs-inactive/interface-js-ipfs-core#125
   - [x] `dag get --localResolve` vs `dag resolve`: This is solved by the new
         `resolve()` API
 - ipfs-inactive/interface-js-ipfs-core#137
   - [x] add `level` option to ipfs.dag.tree: `tree()` is not part of js-ipld
         anymore. It should be considered if `tree()` is implemented (probably
         as an example or separate module)

Closes #182.

Most importantly the `tree()` method is added again. It's really needed
for backwards compatibility.

Loading a format is no longer callback based but is using an
async function.

`doctoc` was used to update the table of contents.

The `format` is no longer part of the the options object, but a
separate parameter. The `version` option was renamed to `cidVersion`.