Auto merge of #23104 - japaric:inherent, r=nikomatsakis

- Allow inherent implementations on `char`, `str`, `[T]`, `*const T`, `*mut T` and all the numeric primitives.
- copy `unicode::char::CharExt` methods into `impl char`
- remove `unicode::char::CharExt`, its re-export `std::char::CharExt` and `CharExt` from the prelude
- copy `collections::str::StrExt` methods into `impl str`
- remove `collections::str::StrExt` its re-export `std::str::StrExt`, and `StrExt` from the prelude
- copy `collections::slice::SliceExt` methods into `impl<T> [T]`
- remove `collections::slice::SliceExt` its re-export `std::slice::SliceExt`, and `SliceExt` from the prelude
- copy `core::ptr::PtrExt` methods into `impl<T> *const T`
- remove `core::ptr::PtrExt` its re-export `std::ptr::PtrExt`, and `PtrExt` from the prelude
- copy `core::ptr::PtrExt` and `core::ptr::MutPtrExt` methods into `impl<T> *mut T`
- remove `core::ptr::MutPtrExt` its re-export `std::ptr::MutPtrExt`, and `MutPtrExt` from the prelude
- copy `core::num::Int` and `core::num::SignedInt` methods into `impl i{8,16,32,64,size}`
- copy `core::num::Int` and `core::num::UnsignedInt` methods into `impl u{8,16,32,64,size}`
- remove `core::num::UnsignedInt` and its re-export `std::num::UnsignedInt`
- move `collections` tests into its own crate: `collectionstest`
- copy `core::num::Float` methods into `impl f{32,64}`

Because this PR removes several traits, this is a [breaking-change], however functionality remains unchanged and breakage due to unresolved imports should be minimal. If you encounter an error due to an unresolved import, simply remove the import:

``` diff
  fn main() {
-     use std::num::UnsignedInt;  //~ error: unresolved import `std::num::UnsignedInt`.
-
      println!("{}", 8_usize.is_power_of_two());
  }
```

---

cc  #16862
[preview docs](http://japaric.github.io/inherent/std/index.html)
[unicode::char](http://japaric.github.io/inherent/unicode/primitive.char.html)
[collections::str](http://japaric.github.io/inherent/collections/primitive.str.html)
[std::f32](http://japaric.github.io/inherent/std/primitive.f32.html)