Replace Convertible instances with new ConvertSqlValue class

Replaced the assorted "Convertible a SqlValue" and
"Convertible SqlValue a" instances with a new ConvertSqlValue
class. Removed a lot of boilerplate in the process.
Remove some boilerplate

Used GHC extensions StandaloneDeriving and
GeneralizedNewtypeDeriving to clean up some trivial ToMarkup
instance declarations.