allow any function to report its toString as a NativeFunction

harness/nativeFunctionMatcher.js

test/built-ins/Function/prototype/toString/AsyncFunction.js

test/built-ins/Function/prototype/toString/AsyncGenerator.js

test/built-ins/Function/prototype/toString/Function.js

...uilt-ins/Function/prototype/toString/GeneratorFunction.js

test/built-ins/Function/prototype/toString/arrow-function.js

...t-ins/Function/prototype/toString/async-arrow-function.js

...Function/prototype/toString/async-function-declaration.js

.../Function/prototype/toString/async-function-expression.js

...unction/prototype/toString/async-generator-declaration.js

...Function/prototype/toString/async-generator-expression.js

...oString/async-generator-method-class-expression-static.js

...otype/toString/async-generator-method-class-expression.js

...toString/async-generator-method-class-statement-static.js

...totype/toString/async-generator-method-class-statement.js

...ction/prototype/toString/async-generator-method-object.js

...rototype/toString/async-method-class-expression-static.js

...ction/prototype/toString/async-method-class-expression.js

...prototype/toString/async-method-class-statement-static.js

...nction/prototype/toString/async-method-class-statement.js