This improves performance of `ncodeunits(::Char)` by simply counting the
number of non-zero bytes (except for `\0`, which is encoded as all zero
bytes). For a performance comparison, see [this gist](
https://gist.github.com/Seelengrab/ebb02d4b8d754700c2869de8daf88cad);
there's an up to 10x improvement here for collections of `Char`, with a
minor improvement for single `Char` (with much smaller spread). The
version in this PR is called `nbytesencoded` in the benchmarks.

Correctness has been verified with Supposition.jl, using the existing
implementation as an oracle:

```julia
julia> using Supposition

julia> const chars = Data.Characters()

julia> @check max_examples=1_000_000 function bytesenc(i=Data.Integers{UInt32}())
           c = reinterpret(Char, i)
           ncodeunits(c) == nbytesdiv(c)
       end;
Test Summary: | Pass  Total  Time
bytesenc      |    1      1  1.0s

julia> ncodeunits('\0') == nbytesencoded('\0')
true
```

Let's see if CI agrees!

Notably, neither the existing nor the new implementation check whether
the given `Char` is valid or not, since the only thing that matters is
how many bytes are written out.

---------

Co-authored-by: Sukera <Seelengrab@users.noreply.github.com>