rustdoc: link implemented methods appropriately

[nodakai]

Example: we see char implements the Char trait which provides encode_utf8():

• http://doc.rust-lang.org/core/char/primitive.char.html#method.encode_utf8

However it isn't easy to browse to its description; Curretnly we must

1. move up to the line of impl Char for char,
2. click Char to open a new page,
3. and then find encode_utf8() in the new page.

We should be able to

1. just click encode_utf8() to open its description without any manual scrolling

Curretnly encode_utf8() in the char page is linked to itself. Perhaps the intention was to provide an easy way for a person reading it with a web browser to copy the link into the OS clipboard. However it is a less likely use case than to try to read the description of each trait method. I think it is convenient enought to link the char part of the impl Char for char line to the same line.