[std::str] Rename from_utf8_owned_opt() to from_utf8_owned(), drop the old from_utf8_owned() behavior

[std::path] Rename .container_as_str_opt() to .container_as_str(), drop the old .container_as_str() behavior

This hasn't been in use since `@mut` was removed

There's lots of fun rationale in the comments of the diff.

Closes #11683

Upgrade the version to 0.10-pre

The implementation has been made more succinct and no longer requires Clone. The coverage of the associated unit test has also been increased to check more combinations of bases, exponents, and expected results.

Zero and One have precise definitions in mathematics. Documentation has been added to describe the appropriate uses for these traits and the laws that they should satisfy.

For more information regarding these identities, see the following wikipedia pages:

- http://wikipedia.org/wiki/Additive_identity
- http://wikipedia.org/wiki/Multiplicative_identity

Signed-off-by: NOGINO Masanori <masanori.ogino@gmail.com>

There was an old and barely used implementation of pow, which expected
both parameters to be uint and required more traits to be implemented.
Since a new implementation for `pow` landed, I'm proposing to remove
this old impl in favor of the new one.

The benchmark shows that the new implementation is faster than the one
being removed:

test num::bench::bench_pow_function               ..bench:      9429 ns/iter (+/- 2055)
test num::bench::bench_pow_with_uint_function     ...bench:     28476 ns/iter (+/- 2202)

Rename existing iterators to get rid of the Iterator suffix and to
give them names that better describe the things being iterated over.

The `malloc` family of functions may return a null pointer for a
zero-size allocation, which should not be interpreted as an
out-of-memory error.

If the implementation does not return a null pointer, then handling
this will result in memory savings for zero-size types.

This also switches some code to `malloc_raw` in order to maintain a
centralized point for handling out-of-memory in `rt::global_heap`.

Closes #11634

This stores the stack of iterators inline (we have a maximum depth with
`uint` keys), and then uses direct pointer offsetting to manipulate it,
in a blazing fast way:

Before:

    bench_iter_large          ... bench:     43187 ns/iter (+/- 3082)
    bench_iter_small          ... bench:       618 ns/iter (+/- 288)

After:

    bench_iter_large          ... bench:     13497 ns/iter (+/- 1575)
    bench_iter_small          ... bench:       220 ns/iter (+/- 91)

This are *trivial* to reimplement in terms of each_reverse if that extra
little bit of performance is needed.

This removes the `Primitive::{bits, bytes, is_signed}` methods and removes the operator trait constraints, for the reasons outlined below:

- The `Primitive::{bits, bytes}` associated functions were originally added to reflect the existing `BITS` and `BYTES` statics included in the numeric modules. These statics are only exist as a workaround for Rust's lack of CTFE, and should probably be deprecated in the future in favor of using the `std::mem::size_of` function (see #11621).

- `Primitive::is_signed` seems to be of little utility and does not seem to be used anywhere in the Rust compiler or libraries. It is also rather ugly to call due to the `Option<Self>` workaround for #8888.

- The operator trait constraints are already covered by the `Num` trait.

* Reexport io::mem and io::buffered structs directly under io, make mem/buffered
  private modules
* Remove with_mem_writer
* Remove DEFAULT_CAPACITY and use DEFAULT_BUF_SIZE (in io::buffered)

The patch adds a `pow` function for types implementing `One`, `Mul` and
`Clone` trait.

The patch also renames f32 and f64 pow into powf in order to still have
a way to easily have float powers. It uses llvms intrinsics.

The pow implementation for all num types uses the exponentiation by
square.

Fixes bug #11499

Fix test to account for new temporary lifetime rules, which cause the channel to be dropped prematurely.

Closes #9582

One less trait in std::num, and three less exported in the prelude.

Major changes:

- Define temporary scopes in a syntax-based way that basically defaults
  to the innermost statement or conditional block, except for in
  a `let` initializer, where we default to the innermost block. Rules
  are documented in the code, but not in the manual (yet).
  See new test run-pass/cleanup-value-scopes.rs for examples.
- Refactors Datum to better define cleanup roles.
- Refactor cleanup scopes to not be tied to basic blocks, permitting
  us to have a very large number of scopes (one per AST node).
- Introduce nascent documentation in trans/doc.rs covering datums and
  cleanup in a more comprehensive way.

WindowsPath::new("C:").join("a") produces r"C:\a". This is incorrect.
It should produce "C:a".

This should allow callers to know whether the channel was empty or disconnected
without having to block.

Closes #11087