Add script and tests for using keywords as identifiers

[fhahn]

This is a pull request for #2275

I've created a small python script to generate test files for a list of keywords (as break do else enum extern false fn for if impl let loop match mod mut priv pub ref return self static struct super true trait type unsafe use while), but I'm not really sure where to put it. I've added the created files as well.

I did not use

 fn main() {
    let $KW = "foo"; //~ error
    println($KW); //~ error
} 

as template, because for return, self, ref, loop, mut and break this does not raise an error in the println line, only in the let line.

[huonw]

I do believe it's 2013. :)

[fhahn]

yes :)

I've updated the date in the copyright notice of the script and use the current year for files created by the script

[pnkfelix]

From past experience with approaches like this: The generated files should include comments saying that they are auto-generated from a script and include the path to the script from the root of the repository. (It might also be good if the generated files were tagged as read-only, where the python script would explicit toggle the write-permission bit back on during the generation process.)

[pnkfelix]

(having said that, thanks for taking the initiative on this!)

[fhahn]

@pnkfelix Thanks for the feedback, I've added the comment to the auto-generated files and set read-only permissions