Expose schema and htable objects in integration tests.

Also change the contract of `.schema`: to pass an inline schema, put
it in a let and pass the let name (as a symbol) to `.schema`.
Families validate that qualifiers can be unambiguously identified.

Askari types are already forward-decodable in isolation, assuming the
decoder is aware of the type.  A decoder that needs to infer the
type can only do so under special circumstances.

A sufficient (but not necessary) way to establish that circumstance is
to require the space of possible types include identifying constants
in such a way that the encoding can only be decoded one way.

A leading disambiguating constant would do this:

    [a, i8]  # Any encoding can decode only to at most one of these
    [b, i8]

However that constraint is maybe a little too tight.  A disambiguating
constant after  a shared prefix of arbitrary, packed type components
would also be sufficient:

    [bytes, a, i8]  # These are also mutually exclusive
    [bytes, b, i8]

This is what we enforce here.
How'd that get there?
Start relation.
Merge specs after all; we need I/O to discover java classpath.
Document a landmine.
RelationSchema#qualifers merges the qualifier maps of its families.
Begin a basic Relation#get.