Fix problems with expressions

[segeljakt]

This PR corrects the grammar so that it is no longer possible to write things like:
!let x = 3; x or for([1,2,3], appender[i32], 5)
The previous expression definition:

expr : letExpr | lambdaExpr | operatorExpr;

Has been changed to:

valueExpr : letExpr | operatorExpr;
expr : lambdaExpr | valueExpr;

This helps rules distinguish lambda expressions from expressions which evaluate to a value. As an example, for-expressions were before defined as:

annotations? TFor '(' iterator ',' builder=expr ',' body=expr ')' # For

and are now:

annotations? TFor '(' iterator ',' builder=valueExpr ',' body=lambdaExpr ')' # For

[Bathtor]

Looking good, thanks :)