Make built-in function source spec compliant.

[aardvark179]

This addresses some of the issues in #1300 by removing the override of decompile on IdFunctionObject so all built in functions print as

function name() {
    [native code]
}

and updating all the doc tests to match this behaviour.

[rbri]

Please run gradlew spotlessAppy to reformat your code..

[aardvark179]

Please run gradlew spotlessAppy to reformat your code..

Sorry about that, it didn't raise the warning locally, but it does it I change the Java version. Hopefully this now passes on all supported versions.

[rbri]

@aardvark179 thanks, looks good now. And no worries, this spotless thing is a source of fun for all...