Improve readability of use-site variance generic expressions for contravariance and covariance

[steveash]

Java's lack of declaration site expression of co/contra-variance dooms us to riddling our use-site generic code with verbose variance expressions that eat up many more characters than they are worth:

Map<String, ? extends Foo<? extends Bar>> myMap = new HashMap<>();

Perhaps a new feature of the Advanced Expression Folding plugin could be to optionally fold these long expressions into just a + or - to indicate covariance or contravariance. The leading plus or minus syntax would follow Scala's use of declaring generic parameter variance:

Map<String, +Foo<+Bar>> myMap = new HashMap<>

[cheptsov]

How about var myMap = new HashMap<String, ? extends Foo<? extends Bar>>()