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

1. A slice of parametrized type, say
   BorrowedRef { ... Vector(Generic(T)) }, is rendered as
   "<a href='primitive.slice.html'>&amp;[T]</a>"
2. A slice of other types, say
   BorrowedRef { ... Vector(int) }, is rendered as
   "<a href='primitive.slice.html'>&amp;[</a>
    <a href='primitive.int.html'>int</a>
    <a href='primitive.slice.html'>]</a>"
3. Other cases, say BorrowedRef { ... int }, are
   rendered as same as before:
   "&<a href='primitive.int.html'>int</a>"

Relevant W3C specs:
- http://www.w3.org/TR/html401/struct/links.html#h-12.2.2
  12.2.2 Nested links are illegal
- http://www.w3.org/TR/html5/text-level-semantics.html#the-a-element
  states A tag must not enclose any "interactive contents"
  which include A tags themselves.
auto merge of #17892 : nodakai/rust/rustdoc-dont-nest-a, r=alexcrichton

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.