Leverage Rust 1.61 to clean up ZeroVec

[sffc]

With Rust 1.61 and https://github.com/unicode-org/icu4x/pull/1926, we will now be able to do the following:

1. Add a general-purpose zeroslice! macro
2. Remove the old type-specific const constructors
3. Migrate users of the problematic EqULE trait to use the new convenience macro, and then remove EqULE

For the macro, you should be able to write

static FOO: &ZeroSlice<u16> = zeroslice![<cb>; 222, 333, 444, 555];

which desugars to

static FOO: &ZeroSlice<u16> = $crate::ZeroSlice::from_ule_slice(
    &<cb>([222, 333, 444, 555])
);

where <cb> is a const function that converts from an aligned to unaligned array (i.e. AsULE::to_unaligned); it would be nice to have a trait for this but we still can't use trait functions in a const context. We can probably write it such that the function only needs to convert from a single T to a single T::ULE and call it in a loop. The big thing is that we can write ZeroSlice::from_ule_slice in a const context now.

[sffc]

Example of using the const function to build a ZeroSlice: https://github.com/unicode-org/icu4x/blob/269b807d9f0eda2905efab6c9e6bd30d8c243a80/utils/codepointtrie/benches/tries/gc_small.rs

The first step for this issue is to migrate all users of from_slice_or_alloc to use the const function if possible.