Adds the ability to use raw SQL in update.

Currently, you cannot add raw SQL to the SET portion of update statements, which prevents generating SQL like SET field1 = field1 + 1, as discussed in Issue 734.

This MR is a suggestion for allowing that, via code like this:

const tests = await db.tests.update(1, {
  $set: {field1: '"field1" + 1'},
});

The above generates the following SQL:

UPDATE "tests SET "field1" = "field1" + 1 WHERE id = 1 RETURNING *

Additionally, attempting to set the same field in the changes map and the $set map will throw an error:

const tests = await db.tests.update(1, {
  $set: {field1: '"field1" + 1'},
  field1: 'value',
});

Returns:

 'The key 'field1' may not be defined in both the change map and the $set map.'