Reserve name `Str` while it's not too late

[petrochenkov]

If str is ever moved to the library, as was planned, it would be good for it to follow the library conventions.
It's likely that even if hypothetical 2.0 allows moderate breaking changes, it will still not be able to use the name Str for practical compatibility reasons. Now (before 1.2), while the language is not widely used, it still might be possible to reserve the name by putting it into the prelude.

Detailed design:

1. Put type Str = str; somewhere in libcore.
2. Reexport Str from the libstd prelude
3. Evaluate the impact with crater
4. ??? (depends on the previous item)

[petrochenkov]

ping @aturon @Gankro
or someone else from the library team?

[petrochenkov]

I've tested the change on Rust itself (branch) - nothing breaks. Name Str is sometimes used, but only in value namespace and not type namespace, so it's not affected.

[petrochenkov]

Ok, it looks like names in the prelude are "weak" and can be redefined in user code without conflicts.

type String = u8; // Compiles, and now String is u8 in the current module

So, putting Str into the prelude later will not be as destructive as it seemed.