Defines a structured type, named /structure-type/, with named slots
as specified by /slot-specs/ and defines a Postmodern row reader named
DEFPGSTRUCT creates and adds to POSTMODERN::*RESULT-STYLES* the
keywords named structure-name and by concatenating with S and !. Those
keywords can be used as result format arguments to POSTMODERN:QUERY,
POSTMODERN:PREPARE, POSTMODERN:DEFPREPARED, and
POSTMODERN:DEFPREPARED-WITH-NAMES. Note that, in doing so, DEFPGSTRUCT
accesses unexported symbols from the POSTMODERN package.
/structure-name/ must be a symbol, like a Common Lisp structure
/structure-name/ rather than a Common Lisp structure /name-and-options/.
Each /slot-spec/ must be either a symbol specifying the slot name or a
list of a symbol specifying the slot name and a desginator for a
function to read and convert the PostgreSQL string value when
initializing the slot value. If /slot-spec/ is a symbol, the slot
value will be initialized to the PostgreSQL string value.