Redefine extra dependency groups

I'm not sure if a firm decision has been made on this but I've heard a few thoughts expressed on the matter:

* Rename "extras" to something more expressive ("tools" or something?)
* Merge the extra dependencies into the main dependencies
* Separate the models/serialization business and the extra tooling into separately installable packages ("core" vs "tools" ?)

edit: had another thought to toss out: move `extras` to a different module level, a la

```
src
└── ga4gh
├── core
├── vrs
└── extras. # or "tools" or w/e
├── __init__.py
├── decorators.py
├── object_store.py
├── translator.py
└── vcf_annotation.py
└── utils
└── hgvs_tools.py
```