Add trait bound styles

[zonyitoo]

1. Add normal trait bound styles
2. Add where clause trait bound styles

[rust-highfive]

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[bluss]

Where does the recommendation to not use where clauses for single bounds come from?

[zonyitoo]

@bluss Because it is short, so you could write them in the front.

[bluss]

If we make style recommendations it must be discussed more widely.

I don't agree with the recommendation here, where clauses should be ok in all cases.

[zonyitoo]

Maybe I should change the word, you could alway use the where clause, but if you decide to write them in the front, then it should be 1-3 type parameters with single bound

[bors]

☔ The latest upstream changes (presumably #29144) made this pull request unmergeable. Please resolve the merge conflicts.

[zonyitoo]

@bluss Is that able to merge? :)

[bluss]

Need wider feedback. By the way, you have some submodule bumps in the PR by mistake, those need to be rebased out.

[steveklabnik]

I don't think we should be modifying the style guide with new recommendations without using the RFC process, or going through the RFC process to define one. As such, I'm going to give this one a close for now. Modifying the style guide will become a really big, important topic once rustfmt is further along.