librustdoc/html: recognize slices not to nest A tags.

Some examples:

- `std::slice::slice`: [before](http://validator.w3.org/check?uri=http%3A%2F%2Fdoc.rust-lang.org%2Fstd%2Fslice%2Fprimitive.slice.html&charset=%28detect+automatically%29&doctype=Inline&group=0) and [after](http://validator.w3.org/check?uri=http%3A%2F%2Fnodakai.github.io%2Frust-f40b60b-doc%2Fstd%2Fslice%2Fprimitive.slice.html&charset=%28detect+automatically%29&doctype=Inline&group=0)
- `core::char::char`: [before](http://validator.w3.org/check?uri=http%3A%2F%2Fdoc.rust-lang.org%2Fcore%2Fchar%2Fprimitive.char.html&charset=%28detect+automatically%29&doctype=Inline&group=0) and [after](http://validator.w3.org/check?uri=http%3A%2F%2Fnodakai.github.io%2Frust-f40b60b-doc%2Fcore%2Fchar%2Fprimitive.char.html&charset=%28detect+automatically%29&doctype=Inline&group=0)
- `hexfloat::expand_syntax_ext`: [before](http://validator.w3.org/check?uri=http%3A%2F%2Fdoc.rust-lang.org%2Fhexfloat%2Ffn.expand_syntax_ext.html&charset=%28detect+automatically%29&doctype=Inline&group=0) and [after](http://validator.w3.org/check?uri=http%3A%2F%2Fnodakai.github.io%2Frust-f40b60b-doc%2Fhexfloat%2Ffn.expand_syntax_ext.html&charset=%28detect+automatically%29&doctype=Inline&group=0)

TODO: do the same for tuples.