Update rust-layers to fix empty BufferRequest failures

rust-layers will now avoid sending empty buffer requests which can
cause failures when Servo tries to create empty platform surfaces.

Fixes #4060.
implement Document#createAttribute
Fixes 4037. Enable backtrace on run cmd.
auto merge of #4067 : wenderen/servo/document-create-attribute, r=Manishearth

fixes #4054
auto merge of #4074 : mrobinson/servo/empty-tiles, r=jdm

rust-layers will now avoid sending empty buffer requests which can
cause failures when Servo tries to create empty platform surfaces.

Fixes #4060.
auto merge of #4075 : nwin/servo/fix-4037, r=Ms2ger

Fixes #4037
Make sure directories aren't getting tested
auto merge of #4072 : frewsxcv/servo/test-files, r=jdm

Fixes #3910
Adds --release flag to ./mach build-cef

Requested by kmc in IRC.

```
11:36 < kmc> while you're mach-ing about, can you modify build-cef to
accept --release like build does?
```
auto merge of #4077 : mttr/servo/mach_build_cef_release, r=kmcallister

Requested by kmc in IRC.

```
11:36 < kmc> while you're mach-ing about, can you modify build-cef to
accept --release like build does?
```
Implement HTML{Anchor,Area,Link}Element.relList.

https://github.com/servo/servo/issues/3994