Support ES2015 function name behavior

[p-bakker]

ES2015 standardized the behavior of the function.name property. Rhino doesn't comply.

See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/name, https://2ality.com/2015/09/function-names-es6.html for explanations of expected behavior. See https://tc39.es/ecma262/#sec-setfunctionname and everywhere in the spec where the abstract function operation setFunctionName is used to see the how and when

Debugger implementations would benefit from this, as they would be able to display more meaningful names on each entry in the callstack

Known issues:

• var x = function(){}; x.name // doesn't yield 'x'
• let f; f = () => {}; f.name // doesn't yield 'f'
• ({x: function test(){}}).x.name // doesn't yield 'x'
• ({foo(){}}).foo.name // doesn't yield 'foo'
• var o = {get foo(){}, set foo(x){}}; var desc = Object.getOwnPropertyDescriptor(o, "foo"); desc.get.name // doesn't yield 'get foo'
• var o = {get foo(){}, set foo(x){}}; var desc = Object.getOwnPropertyDescriptor(o, "foo"); desc.set.name // doesn't yield 'set foo'

Also see #715 for a related case

Some of the scenarios that we currently cannot cover, as they rely on features Rhino doesn't currently support:

• let s = Symbol('someDescription'); ({[s]: () => {})[s].name // to yield [someDescription] Support ES2015 Computed Property Names #913
• default exports, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/name#default-exported_function_declaration Support ES2015 modules #945
• default values in initializers: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/name#initializer_and_default_value Support ES2015 default values inside destructuring assignments #756, Support ES2015 default parameter for functions #678
• method declarations: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/name#shorthand_method Support ES2015 Method Definitions - prevent methods as constructor #1299
• Class properties and private fields ES2015 Classes and inheritance #835

[rbri]

will provide a patch for bound functions - because this is simple to fix :-)