Add SQL.raw, allow queries to be nested

[wolever]

For example:

const columns = SQL.raw("foo, bar");
const whereClause = SQL`foo = ${someValue}`;
const query = SQL`SELECT ${columns} FROM table WHERE ${whereClause}`;

TODO:

• [ ] TypeScript typings
• [ ] Import more extensive unit tests
• [ ] Introduce SQL.join helper:

const fields = {
    foo: 'hello',
    bar: 'world',
}
const updateClause = SQL.join(', ', Object.entries(fields).map(([col, val]) => {
    return SQL`${SQL.raw(escapeLiteral(col))} = ${val}`
})
const query = SQL`UPDATE table SET ${updateClause} WHERE ...`