support direct invocations on grouped expressions

[jvasileff]

Direct invocations on grouped expressions involving unboxed Integers can fail:

shared void run() {
    value a = ("hello ".plus) { "world"; };   // works
    value b = (("hello ".plus)) { "world"; }; // works
    value c = 1.plus { 2; };        // works
    value d = (1.plus) { 2; };      // error: error: long cannot be dereferenced
    value e = ((1.plus)) { 2; };    // error: error: long cannot be dereferenced
    value f = (Singleton)(10);      // works
}

See also ceylon/ceylon-spec#1449

[jvasileff]

Closing (superseded by ceylon/ceylon-spec#1467)