Use `default: D` mirroring Rust documentation

Use `default: D` mirroring Rust documentation

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`.