implement XML input language

[soc]

Next step of #27.

Make the compiler understand *.xsd files, such that

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="person">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="firstName" type="xs:string" />
        <xs:element name="lastName" type="xs:string" />
      </xs:sequence>
      <xs:attribute name="age" type="xs:unsignedInt" use="required" />
    </xs:complexType>
  </xs:element>
</xs:schema>

is accepted as a valid definition of a type Person, that can be instantiated with

<person age="42">
  <firstName>Jane</firstName>
  <lastName>Doe</lastName>
</person>

or

Person(42, "Jane", "Doe")