improved syntax for CRR / CRDT creation

[tantaman]

Allows creation of crrs via virtual table declaration.

Instead of:

CREATE TABLE foo (
  id INT PRIMARY KEY,
  content TEXT,
  other_thing FLOAT
);
SELECT crsql_as_crr('foo');

Users can now:

CREATE VIRTUAL TABLE foo_schema USING CRDT_TYPE (
    id INT PRIMARY KEY,
    content TEXT,
    other_thing FLOAT
);

which will create a table foo as a CRR. foo_schema exists as.. an unfortunate requirement but it has some benefits.

Altering CRRs would now be:

INSERT INTO foo_schema ("ALTER TABLE foo ...");

rather than

SELECT crsql_begin_alter('foo');
ALTER TABLE foo ...
SELECT crsql_end_alter('foo');

Neither of these are ideal but the insertion against the schema table seems less error prone. It could also, maybe, allow more declarative schema management:

UPDATE foo_schema SET schema = "NEW_SCHEMA";

Where we use our auto_migrate plugin to apply the correct schema modifications to update foo to the newly supplied schema.

---

Future iterations will extend this syntax to allow specifying CRDT type on a per-column basis.

Fixes #181

[tantaman]

Note that this does not replace the old APIs of

SELECT crsql_as_crr(...)
and begin/end_alter(...)

This is new syntax for people allergic to mutations done through SELECT and to support later features of column-specific CRDT types (e.g., a counter column or a fugue column).

[tantaman]

You can't alter the CRRs via the virtual tables yet (although the existing alter APIs still work). Will do that in a follow up PR.