Renaming of keywords and types

I would like to see the following syntactic changes. They have to be discussed and rejected or approved.

- rename type `Boolean` to `Bit`
- rename keyword `type` to `datatype`
- keep keyword `type` for type aliases
- change syntax `var x = 1; ...` to `var x := 1; ...`
- change syntax `x = 1; ...` to `x := 1; ...`
- remove `return` statement
- remove `with` statement
- sequence constructors not with semicolons `type B { True(); False() }` but `type B { case True() case False() }`
- create instances without keyword `new` like `def x = State { def get() = 1 def set(y: Int) = () }`
- put the condition in `while` in curly braces `while { cond } { body }`