Add unsafeSqlCast function

[potomak]

It supports a <exp>::<type> cast syntax.

Example usage:

castToNumeric :: UnsafeSqlFunctionArgument a => a -> SqlExpr (Value PersistValue)
castToNumeric = unsafeSqlCast "numeric"

See also

• http://www.postgresql.org/docs/9.4/static/sql-expressions.html#SQL-SYNTAX-TYPE-CASTS
• unsafeSqlExtractSubField to support EXTRACT subfield FROM field-syntax #54

[meteficha]

It's unclear to me how unsafeSqlCast is going to behave when given an argument that toArgList converts to a list with more than one element. Please add unit tests to make sure it works as intended.

Also, can you copy-paste your example into the Haddock? It will help people a lot when looking at this function :). And please add /Since: 2.4.2/ as well.