add Entity default to the entity set wrappers (#18319)

# Objective

Installment of the #16547 series.

The vast majority of uses for these types will be the `Entity` case, so
it makes sense for it to be the default.

## Solution

`UniqueEntityVec`, `UniqueEntitySlice`, `UniqueEntityArray` and their
helper iterator aliases now have `Entity` as a default.

Unfortunately, this means the the `T` parameter for `UniqueEntityArray`
now has to be ordered after the `N` constant, which breaks the
consistency to `[T; N]`.
Same with about a dozen iterator aliases that take some `P`/`F`
predicate/function parameter.
This should however be an ergonomic improvement in most cases, so we'll
just have to live with this inconsistency.

## Migration Guide

Switch type parameter order for the relevant wrapper types/aliases.

Author: Victoronz