Add like and ilike

[ocharles]

I couldn't quite decide the right home for these operators, but have gone for Rel8.Expr.Text in the end. As they are standard SQL operators, I've also re-exported like and ilike from Rel8.

Fixes #144.

[ocharles]

We discussed this and agreed to flip the order of arguments. I'll do that, and add a comment explaining why (it's better for currying)

[bitc]

Something that always bothers me about every ORM I've used is that LIKE/ILIKE take the pattern as type Text.

This always ends up with code that looks like:

productDescription `ilike` ("%" <> searchRequest <> "%")

This is incorrect, because when a user searches for "100% Natural" they now get weird unexpected results. You could almost call this a form of SQL injection!

I think the better approach is to introduce a type called TextPattern or something, that allows safe construction of LIKE/ILIKE patterns (doing proper escaping behind the scenes).

Then the user code would look something like this:

productDescription `ilike` (wildCardMulti <> textFragment searchRequest <> wildCardMulti)

Figuring out how to make this work with dynamic "searchRequest" strings (from the database) might be tricky, but I think is possible