Phs/schema migrations and jiggle format

Iterate on the schema format a little more:

* Notice that versions must be _signed_ one-byte integers, since that's the only kind of one-byte integers we have.
* Add a new structured called  a `Migration`, which records a pair of version numbers and a touch of metadata concerning how one might get from one to the other.  Tiers are each given an optional set of migrations.
* Type names are no longer used for keys.  Instead, tiers form lists (not maps) within tables, and qualifiers form lists within families.  A new attribute to added to each to record the relevant type (`row_key_type` and `qualifier_type` respectively.)  Further, the `type` attribute of a qualifier has been renamed to `value_type` for clarity.  With these changes the type names are parsed along with the rest of the document in one pass of YAML.