Add some const constructors for ULE slices for ZeroVec

[sffc]

In #1644, @makotokato is trying to produce a ZeroVec from include_bytes!(). Currently he uses a transmute to coerce the bytes into a ULE slice. This is not logic that should have to live at the call site.

I think we can't use parse_byte_slice because it's a trait function, which is unfortunate. However, we can solve this on a case-by-case basis by adding const constructors, like

const FOO: &[u16::ULE; 100] = u16::ULE::from_byte_array_const(
    include_bytes!("...")
)
.expect("all bytes are valid for u16::ULE");

CC @Manishearth

[Manishearth]

Hm, this isn't going to work for now:

    Checking zerovec v0.6.0 (/home/manishearth/dev/icu4x/utils/zerovec)
error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
  --> utils/zerovec/src/zerovec/slice.rs:21:6
   |
21 | impl<T> ZeroSlice<T>
   |      ^
...
36 |     pub const fn from_ule_slice(slice: &[T::ULE]) -> &Self {
   |     ------------------------------------------------------ function declared as const here
   |
   = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information

Unfortunately we can't do away with the from_ule_slice() because we need it to be able to construct the zerovec. I can still land the constructor for RawBytesULE though.

[sffc]

I did low-hanging fruit in #1648. The more general case is blocked on upstream.