libcore: remove default to_str implementations for pointer types

These couldn't be overridden and so ended up being quite restrictive. This has
the side effect of changing the stringification of ~vecs, but nothing in
relied on this. Closes #4869.
auto merge of #5098 : pkgw/rust/pr/issue4869, r=brson

See issue #4869. I'm not quite sure what constitutes "consensus from the core team" (cf. discussion in the issue), but this at least demonstrates that the proposed change is pretty straightforward.

After this change, there are no new test failures. I've un-ignored the `to_str` vectors test; it's not at all obvious to me why it'd be problematic, and it passes on my Linux machine.