Implement low-hanging fruit of collection conventions

* Renames/deprecates the simplest and most obvious methods
* Adds FIXME(conventions)s for outstanding work
* Marks "handled" methods as unstable

NOTE: the semantics of reserve and reserve_exact have changed!
Other methods have had their semantics changed as well, but in a
way that should obviously not typecheck if used incorrectly.

Lots of work and breakage to come, but this handles most of the core
APIs and most eggregious breakage. Future changes should *mostly* focus on
niche collections, APIs, or simply back-compat additions.

[breaking-change]
Fallout from collection conventions
Fix a couple of build warnings
link to raw string docs in libregex docs

Fixes #17023
DTSify libserialize traits

- ToBase64
- FromBase64
- ToHex
- FromHex
- ToJson
- Encodable
Implement Default for Box<[T]>
Fix soundness hole in struct with expressions.

Fixes #18567. Struct{x:foo, .. with_expr} did not walk with_expr, which allowed
using moved variables in some cases.  The CFG for structs also built up with
with_expr happening before the fields, which is now reversed. (Fields are now
before the with_expr in the CFG)
Add tests
14.3 - helpfully implied code sample is broken

As a new user, I spent a while confused when flycheck told me the code sample I'd typed in was invalid. I ended up figuring out some of what comes after the code sample more painfully by myself because there was no indication that it was broken in the text beforehand. This one line change makes it clear that the code following it is an experiment that may not work rather than something to assume just works.
mk: Add -C prefer-dynamic to stage3 libs

Right now the windows nightlies are failing because they're encountering a
linker error when producing stage3 libs. The stage3 libs aren't actually used in
general, and we primarily just want to generate a static stage3 binary, not
static stage3 dylibs.
Fixing #18659

Changes the radix multiplier to start at -0.0 instead of -1.0
when reading strings that start with '-'.
rollup merge of #18605 : Gankro/collect-fruit
rollup merge of #18373 : steveklabnik/gh18288
rollup merge of #18555 : jakub-/e-needstest
rollup merge of #18591 : nick29581/dst-bug-str
rollup merge of #18615 : huonw/simd
rollup merge of #18628 : aturon/fixup-error-comment
rollup merge of #18630 : nikomatsakis/purge-the-bars
rollup merge of #18633 : huonw/target-spec-spelling
rollup merge of #18636 : luqmana/armf