Add skip(n: uint) to Read, Seek, File, MemReader and BufferedReader

[nodakai]

Resolve #13989

Note: the second patch is based on UFCS and doesn't compile with snapshot 2014-10-10.

I could write unit tests only for Read::skip(). How should I add more?

[rust-highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[bstrie]

Wow, sorry for the delay!

This needs a rebase, but I wonder if the changes in the past month (like multidispatch traits) would change how #13989 is approached.

[alexcrichton]

@nodakai like @bstrie, I'd like to apologize for the delay here, sorry about that! This week @aturon and I have taken a close look at std::io and all known outstanding issues with the module, and we'll be writing up an RFC for the module quite soon. For now I'm going to close this PR as we're going to discuss the skip issue in the RFC, which should help the outcome of this PR maintain consistency with the rest of the module.

Thanks for being patient, and I'll try to be sure to keep you updated on the status of the RFC (coming soon!).