* Add support for @@isConcatSpreadable in Array.prototype.concat().
* Remove DUK_USE_NONSTD_ARRAY_{CONCAT,MAP}_TRAILER config option support.
The concat option provided real world behavior for a specification "bug"
in ES5.1 but the bug has been fixed in ES2015. The map option was
unnecessary because ES5.0 already had the "non-standard" behavior.
* Add internal helper for OrdinaryHasInstance(), needed by
Function.prototype[@@hasInstance].
* Add internal helper duk_get_method_stridx() which implements the ES2015
GetMethod() specification function.
* Add internal helper duk_to_boolean_top_pop() which occurs in a few
places and shaves some footprint.
* Extend duk_instanceof() and the internal duk_js_instanceof() helper
to include a target @@hasInstance check.
* Add Symbol.hasInstance and Function.prototype[@@hasInstance].
* Add support for non-default attributes for function properties,
needed by Function.prototype[@@hasInstance]. This also fixes
the incorrect attributes of performance.now().
* Better handling of well-known symbol stridx values and their defines.
* Add explicit property attributes for function properties when they
differ from defaults.
* Invalidate the literal cache on every mark-and-sweep round, and allow
collection of pinned literals. This approach still allows the literal
cache to be simplistic and without an invalidation mechanism in normal
operation (between mark-and-sweep rounds), but still allows eventual
collection of pinned literals if they stop occurring.
* Log level fixes.