Update documentation for from_raw_parts

Fixes #19269.
Add test for #18908
Special-case transmute for primitive, SIMD & pointer types.

This detects (a subset of) the cases when `transmute::<T, U>(x)` can be
lowered to a direct `bitcast T x to U` in LLVM. This assists with
efficiently handling a SIMD vector as multiple different types,
e.g. swapping bytes/words/double words around inside some larger vector
type.

C compilers like GCC and Clang handle integer vector types as `__m128i`
for all widths, and implicitly insert bitcasts as required. This patch
allows Rust to express this, even if it takes a bit of `unsafe`, whereas
previously it was impossible to do at all without inline assembly.

Example:

    pub fn reverse_u32s(u: u64x2) -> u64x2 {
        unsafe {
            let tmp = mem::transmute::<_, u32x4>(u);
            let swapped = u32x4(tmp.3, tmp.2, tmp.1, tmp.0);
            mem::transmute::<_, u64x2>(swapped)
        }
    }

Compiling with `--opt-level=3` gives:

Before

    define <2 x i64> @_ZN12reverse_u32s20hbdb206aba18a03d8tbaE(<2 x i64>) unnamed_addr #0 {
    entry-block:
      %1 = bitcast <2 x i64> %0 to i128
      %u.0.extract.trunc = trunc i128 %1 to i32
      %u.4.extract.shift = lshr i128 %1, 32
      %u.4.extract.trunc = trunc i128 %u.4.extract.shift to i32
      %u.8.extract.shift = lshr i128 %1, 64
      %u.8.extract.trunc = trunc i128 %u.8.extract.shift to i32
      %u.12.extract.shift = lshr i128 %1, 96
      %u.12.extract.trunc = trunc i128 %u.12.extract.shift to i32
      %2 = insertelement <4 x i32> undef, i32 %u.12.extract.trunc, i64 0
      %3 = insertelement <4 x i32> %2, i32 %u.8.extract.trunc, i64 1
      %4 = insertelement <4 x i32> %3, i32 %u.4.extract.trunc, i64 2
      %5 = insertelement <4 x i32> %4, i32 %u.0.extract.trunc, i64 3
      %6 = bitcast <4 x i32> %5 to <2 x i64>
      ret <2 x i64> %6
    }

    _ZN12reverse_u32s20hbdb206aba18a03d8tbaE:
    	.cfi_startproc
    	movd	%xmm0, %rax
    	punpckhqdq	%xmm0, %xmm0
    	movd	%xmm0, %rcx
    	movq	%rcx, %rdx
    	shrq	$32, %rdx
    	movq	%rax, %rsi
    	shrq	$32, %rsi
    	movd	%eax, %xmm0
    	movd	%ecx, %xmm1
    	punpckldq	%xmm0, %xmm1
    	movd	%esi, %xmm2
    	movd	%edx, %xmm0
    	punpckldq	%xmm2, %xmm0
    	punpckldq	%xmm1, %xmm0
    	retq

After

    define <2 x i64> @_ZN12reverse_u32s20hbdb206aba18a03d8tbaE(<2 x i64>) unnamed_addr #0 {
    entry-block:
      %1 = bitcast <2 x i64> %0 to <4 x i32>
      %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
      %3 = bitcast <4 x i32> %2 to <2 x i64>
      ret <2 x i64> %3
    }

    _ZN12reverse_u32s20hbdb206aba18a03d8tbaE:
    	.cfi_startproc
    	pshufd	$27, %xmm0, %xmm0
    	retq
Handle conflicting import of items declared in the same module

Fixes #19498
std: change BufWriter to return ShortWrite/EndOfFile
libcore: Fix `Sized` bounds on overloaded function traits.

- Remove the `for Sized?` bound on `core::ops::FnOnce`, as it takes
  `self` by value and can never be implemented by an unsized type.
- Add a missing `Sized?` bound to the blanket `core::ops::FnMut` impl,
  as both `Fn` and `FnMut` are `for Sized?`.
librustc: Don't reuse same alloca for match on struct/tuple field which we reassign to in match body.
collections: dramatically speed up Vec::reserve with magic

Somehow llvm is able to optimize this version of Vec::reserve
into dramatically faster than the old version. In micro-benchmarks
this was 2-10 times faster. It also shaved 14 minutes off of
rust's compile times.

Closes #19281.
Add doctests for keys() and values() of the BTreeMap collection.

Update keys() and values() for BTreeMap based on @Gankro's comments.

Assign keys and values to variables before doing assertion.
fix 404
work around portability issue on FreeBSD, in which the key returned from
pthread_key_create can be 0.
Add doctests for union, diff, sym_diff, and intersection.

Add a rustdoc test for union to exhibit how it is used.

There is already a test for union in the test namespace, but this commit
adds a doctest that will appear in the rustdocs.

Add a doctest for the difference function.

Add a doctest for the symmetric_difference function.

Add a doctest for the intersection function.

Update the union et al. doctests based on @Gankro's comments.

Make the union et al. doctests a bit more readable.
Utilize fewer reexports

In regards to:

https://github.com/rust-lang/rust/issues/19253#issuecomment-64836729

This commit:

* Changes the #deriving code so that it generates code that utilizes fewer
  reexports (in particur Option::* and Result::*), which is necessary to
  remove those reexports in the future
* Changes other areas of the codebase so that fewer reexports are utilized
Register new snapshots
prefer "FIXME" to "TODO".
librustc: Fix debuginfo for captured variables in non-FnOnce unboxed closures.
Implement BitOps for TreeSet
Correct minor typos on the ownership guide.

liftimes -> lifetimes
Typo
Feature-gate explicit unboxed closure method calls & manual impls,
detect UFCS drop and allow UFCS methods to have explicit type parameters.

Work towards #18875.

Since code could previously call the methods & implement the traits
manually, this is a

[breaking-change]

Closes #19586. Closes #19375.