Boolean DB type casting extension

[mathemancer]

Fixes #387

This adds some strings that can be cast to boolean:

• yes -> true
• no -> false
• y -> true
• n -> false
• on -> true
• off -> false

Technical details

Note that this does not restrict columns to have only two values for inferring boolean. So, a column with '0', '1', 'true', 'TRUE', 'yes', 'off', ... would be inferred to be of boolean DB type. This seems like the best compromise to make at the moment.

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[silentninja]

Looks good to me!

[silentninja]

@mathemancer I am able to cast string values('on', 'y') using the default cast, SELECT 'on'::bool' works, any reason why you didn't go for that?. I approved thinking that was not possible

[mathemancer]

@silentninja It is possible for all strings we do want to handle. To control things a bit, I wanted to set aside separate "approved" strings (to avoid, e.g., 'of' -> false). For numbers, we need the custom behavior since the ways and types of numbers cast to booleans are a bit confusing in their default state. I'm open to changing the behavior if we want later, but I think the current setup is the best compromise with the current constraints.

[silentninja]

Makes sense, seems extensible too.