spring cleaning

[stefanpenner]

I can gladly split this up by file, or by module or by folder or leave it as is. Open to recommendations.

• use let/const when appropriate (and according to style guide) no more var
• concise functions when appropriate
• use => were appropriate

[Turbo87]

I see a lot of conflicts with #13549 ahead of me... 😄

[stefanpenner]

I see a lot of conflicts with #13549 ahead of me... 😄

I can rebase that for you once this lands

[Turbo87]

don't worry, I can take it 😉

[Turbo87]

I'd appreciate an r+ on emberjs/emberjs-build#163 though