Overhauls import expressions
Fixes existential clauses
Refines identifier productions

Needed to also support expressions of the form foo+bar
Renames some expressions
Fixes Op restrictions

Did not allow expressions of the form +foo
Makes first identifier for Super path optional
Tweaks expressions

Nicer AST:
  * No more all PrefixExpr without a prefix
  * ";" for if … else is no longer visible in AST
  * PostfixArgumentExprs is now called SequenceArgumentExprs
Adds own production for assignments

* removes ambiguities between = and =>
Type annotation must be on same line as type

According to the reference, there may be a newline after an annotation.
The case of interest is the following:

def someMethod: A
@annotation
def anotherMethod: B

Here the annotation should be part of the anotherMethod signature and
not for A.
Some changes to get a nicer AST
Adds productions for "val" and "var"

Removes ambiguities in constructors.

Constr(value: Bar) could have been parsed as
Constr(val ue: Bar) before
Adds some priorities to remove ambiguities