RecordType updates

[GuntherRademacher]

This PR consists of three commits:

• bc3ad9f: fixes instance of for maps vs. records: maps are no longer treated as instances of record types without validating required fields and their types. This fixes an incorrect result for cases like:

  map:entries({ 'x': 1 }) instance of record(a)

• f7073c5: implements the previously missing intersection between records and maps, fixing the wrong result that occurred for

  let $map := (
    {'x':5, 'y':6}
    => map:put(xs:NCName('x'), true())
    => map:put(xs:NCName('y'), (false(), false()))
  )
  return (
    $map instance of map(xs:NCName, xs:boolean+) or
    $map instance of map(xs:string, xs:boolean+)
  )

• f191685: removes the extensible flag from records and adapts record coercion to drop excess fields.