Add Slice#unsafe_slice_of, #to_unsafe_bytes

[HertzDevil]

Resolves #11010 , including making #hexdump and #hexstring work for arbitrary Slices.

[oprypin]

Hm sorry, of all these things I like only the addition of unsafe_slice_of. It is a frequent operation and is marked as unsafe so that's fine.

• to_unsafe_bytes is just an alias of unsafe_slice_of(UInt8), not worth the mental overhead.

I would actually put messing with endianness into unsafe territory as well, so my following reasonings are with that in mind:

• to_bytes returns a system-dependent value without directly acknowledging it in its name. Instead I would suggest an equivalent to it in the docs of to_unsafe_bytes. Which, by the way, you do as Slice.new(to_unsafe.as(UInt8*), bytesize, read_only: true) but couldn't it instead be to_unsafe_bytes.dup?

• Changing hexstring to become an alias of to_unsafe_bytes.hexstring and returning a system-dependent value is definitely not good. Just suggest the latter piece of code in the docs of hexstring.

[oprypin]

And, well, seeing as my latter two items hinge on the existence of to_unsafe_bytes, maybe that one is actually fine.