Test / Plug more BCL classes

[fanoI]

Verify (adding tests to TestRunner) that those classes are working in Cosmos and if not plug the needed methods:

• StringBuilder
• StringReader
• StringWriter
• XmlReader
• XmlWriter

[quajak]

Both XmlReader and XMlWriter require extensive plugging as they depend on threading and require improvements in IL2CPU so they can not currently be plugged.