move RawSequences to q2-types, rename to MultiplexedSequences

[jairideout]

Improvement Description
RawSequences is a pretty common/important type so it makes sense to move to q2-types. It would be great to rename it to something like MultiplexedSequences (or possibly a more specific name) since we already have the concept of demultiplexed sequences, and "raw sequences" isn't very informative of a name.

Proposed Behavior
When this move and rename happens, we'll need to handle it in a backward-compatible way so that existing artifacts with this type will continue to work. I think this is the first case of a type move/rename that needs backward-compatibility.

[ebolyen]

Moving shouldn't be too much of an issue, but renaming will be. I think we need an aliasing mechanism for types and file/directory formats.

[gregcaporaso]

Replaced with #153.