Generate documentation for structures (#47)

* Generate documentation for structures

The first of many commits to generate docs. This is a first pass for structures, we still need to document enums & unions. Some serious design also needs to occur to figure out the best practice for turning the Smithy documentation into nice Rust documentation.

* Bump Java version to 9

* Remove exclusion of `target`

* Don't build docs for deps

* Update to use getMemberTrait