Support for SYSTEM TABLE, SYSTEM VIEW types in table_info

Issue
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`table_info` as an *object search* (`table_info('', '%', '%')`) returns SYSTEM TABLE and SYSTEM VIEW objects.  However, it does _not_ support searching for these types of objects.  Instead, a SYSTEM FOO type specifier matches a FOO object or a SYSTEM FOO object.

Related, `table_info` as a *type search* (`table_info('', '', '', '%')`) does not recognize the existence of SYSTEM TABLEs and SYSTEM VIEWs at all.

Proposed Resolution
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This pull request modifies Pg.pm to:

  1.  Return SYSTEM TABLE and SYSTEM VIEW as _bona fide_ types known to `table_info('', '', '', '%')`
  2.  Permit explicit searching for these types which does not conflate with searching for plain TABLE and VIEW objects.

Additionally t/03dbmethod.t is modified to add a few test cases.  In particular, we test
that all supported types are returned by the "type search":

    not ok 97 - DB handle method "table_info" type inquiry returns all supported types
    
    #   Failed test 'DB handle method "table_info" type inquiry returns all supported types'
    #   at t/03dbmethod.t line 556.
    #     Structures begin differing at:
    #          $got->[0] = 'TABLE'
    #     $expected->[0] = 'SYSTEM TABLE'

(Of course that test passes with these changes.)