Use default: D mirroring Rust documentation

[tcharding]

Description

Currently we use F: f for the argument that is the default function passed to map_or_else and pass a closure for the second argument. This bent my brain while reading the documentation because the docs use default: D for the first and f: F for the second. Although this is totally trivial it makes deciphering the combinator chain easier if we name the arguments the same way the Rust docs do.

Use default: D for the identifier of the default function passed into map_or_else.

ref: https://doc.rust-lang.org/std/result/enum.Result.html#method.map_or_else

pub fn map_or_else<U, D, F>(self, default: D, f: F) -> U where
    F: FnOnce(T) -> U,
    D: FnOnce(E) -> U, 

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