Revamp doc index

Fixes #14972
Fix vector/array/slice terminology in manual.

Fixes #16015.
enable jemalloc debugging in unoptimized builds

The performance hit from these checks is significant, but unoptimized
builds are already incredibly slow. Enabling these checks results in
better test coverage since there are bots doing unoptimized builds, and
the cost is relatively small in the context of an unoptimized build.
This also allows using `JEMALLOC_FLAGS` to override the default
configure flags.
rand: inform the optimiser that indexing is never out-of-bounds.

This uses a bitwise mask to ensure that there's no bounds checking for
the array accesses when generating the next random number. This isn't
costless, but the single instruction is nothing compared to the branch.

A `debug_assert` for "bounds check" is preserved to ensure that
refactoring doesn't accidentally break it (i.e. create values of `cnt`
that are out of bounds with the masking causing it to silently wrap-
around).

Before:

    test test::rand_isaac   ... bench: 990 ns/iter (+/- 24) = 808 MB/s
    test test::rand_isaac64 ... bench: 614 ns/iter (+/- 25) = 1302 MB/s

After:

    test test::rand_isaac   ... bench: 877 ns/iter (+/- 134) = 912 MB/s
    test test::rand_isaac64 ... bench: 470 ns/iter (+/- 30) = 1702 MB/s

(It also removes the unsafe code in Isaac64Rng.next_u64, with a *gain*
in performance; today is a good day.)
Update LLVM to fix a crash in the MergeFunc pass
auto merge of #16825 : steveklabnik/rust/fix_manual_array_terms, r=brson

fixes #16015
auto merge of #17083 : thestinger/rust/jemalloc, r=alexcrichton

The performance hit from these checks is significant, but unoptimized
builds are already incredibly slow. Enabling these checks results in
better test coverage since there are bots doing unoptimized builds, and
the cost is relatively small in the context of an unoptimized build.
This also allows using `JEMALLOC_FLAGS` to override the default
configure flags.
auto merge of #16965 : huonw/rust/isaac-oob--, r=alexcrichton

rand: inform the optimiser that indexing is never out-of-bounds.

This uses a bitwise mask to ensure that there's no bounds checking for
the array accesses when generating the next random number. This isn't
costless, but the single instruction is nothing compared to the branch.

A `debug_assert` for "bounds check" is preserved to ensure that
refactoring doesn't accidentally break it (i.e. create values of `cnt`
that are out of bounds with the masking causing it to silently wrap-
around).

Before:

test test::rand_isaac   ... bench: 990 ns/iter (+/- 24) = 808 MB/s
test test::rand_isaac64 ... bench: 614 ns/iter (+/- 25) = 1302 MB/s

After:

test test::rand_isaac   ... bench: 877 ns/iter (+/- 134) = 912 MB/s
test test::rand_isaac64 ... bench: 470 ns/iter (+/- 30) = 1702 MB/s

(It also removes the unsafe code in Isaac64Rng.next_u64, with a *gain*
in performance; today is a good day.)
auto merge of #16827 : steveklabnik/rust/fix_doc_index, r=brson

Fixes #14972
auto merge of #16927 : dotdash/rust/llvmup, r=alexcrichton
fixes for Box<[T]>

The pointer in the slice must not be null, because enum representations
make that assumption. The `exchange_malloc` function returns a non-null
sentinel for the zero size case, and it must not be passed to the
`exchange_free` lang item.

Since the length is always equal to the true capacity, a branch on the
length is enough for most types. Slices of zero size types are
statically special cased to never attempt deallocation. This is the same
implementation as `Vec<T>`.

Closes #14395