use "var" and "val" literals, fix layout constraints for infix expressions
Merge branch 'master' of github.com:fjak/scala-grammar

Conflicts:
	syntax/Scala-Expressions.sdf
	syntax/Scala-Identifiers.sdf
fixing identifiers
Fixes prefix operators
Refines identifiers and symbols

* symbols can now be keywords again, e.g. 'this
* foo + foo_++(42) is now correctly parsed as
  Infix(foo, +, App(foo_++, 42))
* identifiers can start and end with underscores and can have
  underscores in between
* __ is recognized as valid identifier