Rollup merge of #80720 - steffahn:prettify_prelude_imports, r=camelid,jyn514
Rollup merge of #80720 - steffahn:prettify_prelude_imports, r=camelid,jyn514

Make documentation of which items the prelude exports more readable.

I recently figured out that rustdoc allows link inside of inline code blocks as long as they’re delimited with `<code> </code>` instead of `` ` ` ``. I think this applies nicely in the listing of prelude exports [in the docs](https://doc.rust-lang.org/std/prelude/index.html). There, currently unformatted `::` and `{ , }` is used in order to mimick import syntax while attatching links to individual identifiers.

## Rendered Comparison
### Currently (light)
![Screenshot_20210105_155801](https://user-images.githubusercontent.com/3986214/103661510-1a87be80-4f6f-11eb-8360-1dfb23f732e8.png)

### After this PR (light)
![Screenshot_20210105_155811](https://user-images.githubusercontent.com/3986214/103661533-1f4c7280-4f6f-11eb-89d4-874793937824.png)

### Currently (dark)
![Screenshot_20210105_155824](https://user-images.githubusercontent.com/3986214/103661571-2a9f9e00-4f6f-11eb-95f9-e291b5570b41.png)

### After this PR (dark)
![Screenshot_20210105_155836](https://user-images.githubusercontent.com/3986214/103661592-2ffce880-4f6f-11eb-977a-82afcb07d331.png)

### Currently (ayu)
![Screenshot_20210105_155917](https://user-images.githubusercontent.com/3986214/103661619-39865080-4f6f-11eb-9ca1-9045a107cddd.png)

### After this PR (ayu)
![Screenshot_20210105_155923](https://user-images.githubusercontent.com/3986214/103661652-3db26e00-4f6f-11eb-82b7-378e38f0c41f.png)

_Edit:_ I just noticed, the “current” screenshots are from stable, so there are a few more differences in the pictures than the ones from just this PR.

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust