Forbid the Sized bound on unsized types

closes #16800
Rename `RawPtr::to_option()` to `RawPtr::as_ref()`

As outlined in

  https://aturon.github.io/style/naming/conversions.html

`to_` functions names should only be used for expensive operations.
Thus `to_option` is better named `as_option`. Also, putting type
names into method names is considered bad style; what the user is
really trying to get is a reference. This `as_ref` is even better.

Also, we are missing a mutable version of this method. So add a
new trait `RawMutPtr` with a corresponding `as_mut` methode.

Finally, there is a bug in the signature of `to_option` which has
been around since lifetime elision: originally the returned reference
had 'static lifetime, but since the elision changes this become
the lifetime of the raw pointer (which does not make sense, since
the pointer lifetime and referent lifetime are unrelated). Fix
the bug to return a reference with a fresh lifetime (which will
be inferred from the calling context).

[breaking-change]
Make doc search results use <a> tags instead of js for navigating

This has the primary advantage of not interfering with browser default
behavior for links like being able to cmd/ctrl+click on a result to open
the result in a new tab but leave the current page as-is (previous
behavior both opened a new tab and changed the current tab's location to
the result's)
Second approach - using type contents
Fix MIPS target

Add missing liblibc constants
Fix ICE when casting `&[T]` to `*const [T]`

Closes #16947
Handle custom deref returning fat pointers

Closes #16930
Enable a test for correct treatment of Sized?
bitv: add larger tests, better benchmarks & remove dead code.

There were no tests for iteration etc. with more than 5 elements,
i.e. not even going beyond a single word. This situation is rectified.

Also, the only benchmarks for `set` were with a constant bit value,
which was not indicative of every situation, due to inlining & branch
removal. This adds a benchmark at the other end of the spectrum: random
input.
auto merge of #16956 : nick29581/rust/unsized-test-1, r=alexcrichton

I landed this disabled, but it now passes since @pcwalton implemented RFC 11

r?
auto merge of #16634 : apoelstra/rust/to-option-fix, r=aturon

As outlined in

  https://aturon.github.io/style/naming/conversions.html

`to_` functions names should only be used for expensive operations.
Thus `to_option` is better named `as_option`. Also, putting type
names into method names is considered bad style; what the user is
really trying to get is a reference. This `as_ref` is even better.

Also, we are missing a mutable version of this method.

Finally, there is a bug in the signature of `to_option` which has
been around since lifetime elision: originally the returned reference
had 'static lifetime, but since the elision changes this become
the lifetime of the raw pointer (which does not make sense, since
the pointer lifetime and referent lifetime are unrelated). We fix
the bug to return a reference with a fresh lifetime which will be
inferred from the calling context.

[breaking-change]
auto merge of #16811 : nick29581/rust/dst-bug-2, r=nikomatsakis

closes #16800 
r? @nikomatsakis - I'm not 100% sure this is the right approach, it is kind of ad-hoc. The trouble is we don't have any intrinsic notion of which types are sized and which are not, we only have the Sized bound, so I have nothing to validate the Sized bound against.
libtime: Fix adding negative duration to Timespec.
auto merge of #16932 : crabtw/rust/mips, r=alexcrichton

Add missing liblibc constants
auto merge of #16953 : nick29581/rust/dst-bug-7, r=pcwalton

Closes #16947 

r?
auto merge of #16954 : nick29581/rust/dst-bug-deref, r=nikomatsakis

Closes #16930 

r?
auto merge of #16961 : huonw/rust/bitv-twiddle, r=alexcrichton

bitv: add larger tests, better benchmarks & remove dead code.

There were no tests for iteration etc. with more than 5 elements,
i.e. not even going beyond a single word. This situation is rectified.

Also, the only benchmarks for `set` were with a constant bit value,
which was not indicative of every situation, due to inlining & branch
removal. This adds a benchmark at the other end of the spectrum: random
input.
auto merge of #16972 : ruud-v-a/rust/timespec-arithmetic, r=alexcrichton

Changing from `Timespec` to `Duration` introduced a bug for negative durations. This fixes that.
auto merge of #16909 : carols10cents/rust/docs-links, r=alexcrichton

This has the primary advantage of not interfering with browser default behavior for links like being able to cmd/ctrl+click on a result to open the result in a new tab but leave the current page as-is (previous behavior both opened a new tab and changed the current tab's location to the result's).

I've done my best to keep the rest of the behavior and the appearance the same-- the whole row still highlights, still has a hand cursor, still moves to the result page with a normal click, arrows+enter still work. If the result is on the current page, the search is simply hidden.

The biggest difference in behavior is that people using tab to navigate through the links will have to hit tab twice for each row, since each cell has its own `a` tag.. I could fix this by switching to `div`s and `span`s instead of a table, but that's potentially more CSS finicky?

The biggest difference in appearance is probably that all the text in the search results is Fira Sans now, instead of just the method name with the rest of the text in Source Serif Pro. I can put this appearance back, but it looks like all links anywhere on the page are Fira Sans. Only the name was in an `a` tag before, but the whole row was ACTING like a link, so I think this is actually more consistent.

[I've pushed these changes to a gh-pages repo](https://carols10cents.github.io/rustdoc-playground/std/index.html?search=t) if you'd like to take a look at the effects; note that I also have my changes for PR #16735 there too so the search results will be sorted differently than on master.