std: Second pass stabilization for `string`

This commit performs a second pass over the `std::string` module, performing the
following actions:

* The name `std::string` is now stable.
* The `String::from_utf8` function is now stable after having been altered to
  return a new `FromUtf8Error` structure. The `FromUtf8Error` structure is now
  stable as well as its `into_bytes` and `utf8_error` methods.
* The `String::from_utf8_lossy` function is now stable.
* The `String::from_chars` method is now deprecated in favor of `.collect()`
* The `String::from_raw_parts` method is now stable
* The `String::from_str` function remains experimental
* The `String::from_raw_buf` function remains experimental
* The `String::from_raw_buf_len` function remains experimental
* The `String::from_utf8_unchecked` function is now stable
* The `String::from_char` function is now deprecated in favor of
  `repeat(c).take(n).collect()`
* The `String::grow` function is now deprecated in favor of
  `.extend(repeat(c).take(n)`
* The `String::capacity` method is now stable
* The `String::reserve` method is now stable
* The `String::reserve_exact` method is now stable
* The `String::shrink_to_fit` method is now stable
* The `String::pop` method is now stable
* The `String::as_mut_vec` method is now stable
* The `String::is_empty` method is now stable
* The `IntoString` trait is now deprecated (there are no implementors)
* The `String::truncate` method is now stable
* The `String::insert` method is now stable
* The `String::remove` method is now stable
* The `String::push` method is now stable
* The `String::push_str` method is now stable
* The `String::from_utf16` function is now stable after its error type has now
  become an opaque structure to carry more semantic information in the future.

A number of these changes are breaking changes, but the migrations should be
fairly straightforward on a case-by-case basis (outlined above where possible).

[breaking-change]