Add support for CHECK constraints

[JanJakes]

Implements support for CHECK constraints:

CREATE TABLE t (
	id INT NOT NULL CHECK (id > 0),
	name VARCHAR(255) NOT NULL CHECK (name != ''),
	score DOUBLE NOT NULL CHECK (score > 0 AND score < 100),
	data JSON CHECK (json_valid(data)),
	start_timestamp TIMESTAMP NOT NULL,
	end_timestamp TIMESTAMP NOT NULL,
	CONSTRAINT c1 CHECK (id < 10),
	CONSTRAINT c2 CHECK (start_timestamp < end_timestamp),
	CONSTRAINT c3 CHECK (length(data) < 20)
)

Resolves #251.