Updated RELEASES.md for 1.21.0

[Rendered](https://github.com/Aaronepower/rust/blob/master/RELEASES.md)

incr.comp.: Switch to red/green change tracking, remove legacy system.

This PR finally switches incremental compilation to [red/green tracking](https://github.com/rust-lang/rust/issues/42293) and completely removes the legacy dependency graph implementation -- which includes a few quite costly passes that are simply not needed with the new system anymore.

There's still some documentation to be done and there's certainly still lots of optimizing and tuning ahead -- but the foundation for red/green is in place with this PR. This has been in the making for a long time `:)`

r? @nikomatsakis
cc @alexcrichton, @rust-lang/compiler

Remove mem::transmute used in Box<str> conversions

Given that https://github.com/rust-lang/rust/pull/44877 is failing, I decided to make a separate PR. This is done with the same motivation: to avoid `mem::transmute`-ing non `#[repr(C)]` types.

add notes to report_conflicting_borrow MIR borrowck

part of #44596

Update docs for process::abort

Remove a typo and explain the relationship to `panic!`.

Part of #29370

r? @steveklabnik

Overlapping borrows can point to different lvalues.

Overlapping borrows can point to different lvalues.

There's always a basis for the overlap, so instead of removing the assert entirely, I instead pass in the prefix that we found and check that it actually is a prefix of both lvalues.

Fix #44829

make `backtrace = false` compile for windows targets.

when building for windows with `backtrace = false`, `libstd` fails to compile because some modules that use items from `sys_common::backtrace::*` are still included, even though those modules aren't used or referenced by anything.

`sys_common::backtrace` doesn't exist when the backtrace feature is turned off.

--

i've also added `#[cfg(feature = "backtrace")]` to various items that exist exclusively to support `mod backtrace` since the compilation would fail since they would be unused in a configuration with backtraces turned off.

Made `fs::copy` return the length of the main stream

On Windows with the NTFS filesystem, `fs::copy` would return the sum of the
lengths of all streams, which can be different from the length reported by
`metadata` and thus confusing for users unaware of this NTFS peculiarity.

This makes `fs::copy` return the same length `metadata` reports which is the
value it used to return before PR #26751. Note that alternate streams are still
copied; their length is just not included in the returned value.

This change relies on the assumption that the stream with index 1 is always the
main stream in the `CopyFileEx` callback. I could not find any official
document confirming this but empirical testing has shown this to be true,
regardless of whether the alternate stream is created before or after the main
stream.

Resolves #44532

let htmldocck.py check for directories

Since i messed this up during https://github.com/rust-lang/rust/pull/44613, i wanted to codify this into the rustdoc tests to make sure that doesn't happen again.

MIR borrowck: move span_label to `borrowck_errors.rs`

The calls to `span_label` are moved and factorized for:
* E0503 (`cannot_use_when_mutably_borrowed()`)
* E0506 (`cannot_assign_to_borrowed()`)

Additionnally, the error E0594 (`cannot_assign_static()`) has been factorized between `check_loan.rs` and `borrowc_check.rs`.

Part of #44596

Move monomorphize::resolve() to librustc

this moves `monomorphize::resolve(..)` to librustc, and re-enables inlining for some trait methods, fixing #44389

@nikomatsakis I've kept the calls to the new `ty::Instance::resolve(....)` always `.unwrap()`-ing for the moment, how/do you want to add more debugging info via `.unwrap_or()` or something like this?

we still have some related `resolve_*` functions on monomorphize, but I wasn't sure moving them was into the scope for this PR too.

@EddyB mind to take a look too?

There's always a basis for the overlap, so instead of removing the
assert entirely, I instead pass in the prefix that we found and check
that it actually is a prefix of both lvalues.

Fix #44829

 rustdoc: Render [src] links for trait implementors

Should close #43893.

<s>No tests [yet].</s>

r? @QuietMisdreavus

make non_snake_case lint allow extern no-mangle functions

Resolves #31924.

r? @sfackler

update mdbook

Fixes #44704

More Altivec Intrinsics

Float-specific intrinsics

Fixes #44704

per @GuillaumeGomez's sample, but with one change.

handle nested generics in Generics::type_param/region_param

Fixes #44952.

r? @EddyB

code suggestions for unused-mut, while-true, deprecated-attribute, and unused-parens lints

![lint_suggestions](https://user-images.githubusercontent.com/1076988/31044068-b2074de8-a57c-11e7-9319-6668508b6d1f.png)

r? @estebank

Resolves #36284 - vec.rs documentation

Removed comments associated with `[into_vec]` being equivalent to `[shrink_to_fit]` as requested.

Resolves #31924.