Fix new instance creation

[valdisxp1]

A new instance is created by applying the constructor method <init> to the arguments, not by calling New on the type applied to the arguments. I found this out by running the parser on new Some[Int](3), i.e. a modified version of dotty/tools/dotc/parsing/parseFile.scala.

• Created a new constructor NewInstance
• Combined New and AnonymClass into NewAnonymClass as discussed in #73
• Refactor: decomposed liftInitCall into multiple always successful, private extractors. This way we do not have to have 2^n cases in a match statement. Reused the same extractors for liftNewInstance
• Added tests for new instance creation

Necessary fix for JSON deserialization #41

[liufengyun]

Thanks a lot @valdisxp1 , I like this change!