Suggest `Entry::or_default` for `Entry::or_insert(Default::default())`

Unlike past similar work done in #6228, expand the existing `or_fun_call`
lint to detect `or_insert` calls with a `T::new()` or `T::default()`
argument, much like currently done for `unwrap_or` calls. In that case,
suggest the use of `or_default`, which is more idiomatic.

Note that even with this change, `or_insert_with(T::default)` calls
aren't detected as candidates for `or_default()`, in the same manner
that currently `unwrap_or_else(T::default)` calls aren't detected as
candidates for `unwrap_or_default()`.

Also, as a nearby cleanup, change `KNOW_TYPES` from `static` to `const`,
since as far as I understand it's preferred (should Clippy have a lint
for that?).

Fixes #3812.

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