Add class-based schema definitions

[evansd]

This PR replaces the old syntax for defining ehrQL schemas e.g.

patients = build_patient_table(
    "patients",
    {
        "sex": str,
        "date_of_birth": datetime.date,
    }
)

with a class-based one that looks like this:

@construct
class patients(PatientFrame):
    sex = Series(str)
    date_of_birth = Series(datetime.date)

There are some obvious peculiarities here (the decorator and the lowercase class name) which we'll come to later, but the primary motivation for this change is to make it possible to have full autocomplete for ehrQL when writing dataset definitions in VSCode. As far as I can see, this is only possible if we use classes to define the schema because no other syntax makes the column names available.

This change gives us autocomplete on table and column names, and also on frame methods like exists_for_patient, take and drop. It doesn't yet give us autocomplete on series methods but I have a proof-of-concept which shows that this ought to be possible.

The @construct decorator, and the lowercase class name are to avoid having to write something like this:

class Patients(PatientFrame):
    __tablename__ = "patients"
    sex = Series(str)
    date_of_birth = Series(datetime.date)

patients = Patients()

Not only would that be more verbose and error prone, but it would leave the Patients class in the namespace where it would show as a (misleading) autocomplete option.

[iaindillingham]

This looks much nicer 🙂