Consolidate Result's and Option's methods into fewer impl blocks

[dtolnay]

Result's and Option's methods have historically been separated up into impl blocks based on their trait bounds, with the bounds specified on type parameters of the impl block. I find this unhelpful because closely related methods, like unwrap_or and unwrap_or_default, end up disproportionately far apart in source code and rustdocs:

impl<T> Option<T> {
    pub fn unwrap_or(self, default: T) -> T {
        ...
    }

    
}

impl<T: Default> Option<T> {
    pub fn unwrap_or_default(self) -> T {
        ...
    }
}

I'd prefer for method to be in as few impl blocks as possible, with the most logical grouping within each impl block. Any bounds needed can be written as where clauses on the method instead:

impl<T> Option<T> {
    pub fn unwrap_or(self, default: T) -> T {
        ...
    }

    pub fn unwrap_or_default(self) -> T
    where
        T: Default,
    {
        ...
    }
}

Warning: the end-to-end diff of this PR is computed confusingly by git / rendered confusingly by GitHub; it's practically impossible to review that way. I've broken the PR into commits that move small groups of methods for which git behaves better — these each should be easily individually reviewable.

[rust-highfive]

r? @joshtriplett

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[joshtriplett]

Looking at the UI test updates, I like the net effect of this on the error messages.

@bors r+

[bors]

📌 Commit 5960f7a has been approved by joshtriplett