Make Error.stack faster.

Make Error.stack faster.

Previously, error.stack reported full names for native function (for
example Array.prototype.map -> map), but it was achieved by iteration
through a global object which is slow.

The fix is to report only function name at hand, this loses a bit
of verbosity but make it ~100 times faster.

make benchmark
before
        ...
	exception.stack: 38.421µs, 26027 times/s
	exception.native.stack: 226.711µs, 4410 times/s
after
        ...
	exception.stack: 1.239µs, 807356 times/s
	exception.native.stack: 2.419µs, 413339 times/s