Feat siu dispatch

[machow]

Refactor of siu internals to support

• singledispatch as the mechanism for translating sql column methods
• the concept of codata (alternative methods for the same data) for sql dialects, by passing objects like PostgresqlColumn() as the first argument.
  • E.g. mean(PostgresqlColumn(), sqlalchemy.sql.column("x"))
  • E.g. mean(PostgresqlColumnAgg(), sqlalchemy.sql.column("x"))

As a necessary step, refactor the pulling of test specifications (E.g. siuba.ops.support.base), to:

• make it easier to ask a generic function what data it supports (e.g. PostgresqlColumn)
• make it easier to ask a generic function what limits come with dispatching on a piece of data (e.g. dispatching mean(PostgresqlColumn(), column('x')) might return a float, when pandas returns an integer, or vv).