`lang::xml::IO.readXML` is missing a validation feature

[jurgenvinju]

Describe the bug

• add an alternative readXML(type[&T] expected, loc src) and readXML(type[&T] expected, src src)
• use the grammar to call vf.constructor(..) rather than vf.node where possible and throw exceptions if the xml does not fit the grammar
• similar to lang::json::IO