Add interning to Bytes, reducing the need for allocations when slices are small.

[p-avital]

The last bit of the memory layout is the determinant for interning:

• If it's unset, the layout is as expressed from the struct definition, and the pointer is guaranteed to be a valid reference to a vtable. No legal reference to a vtable can set this bit, as it would infringe on alignment constraints.
• If the bit is set, the layout is interned: the last byte (shifted by 1) is used as the length of the slice, and the slice starts at the first byte of the memory layout.

Interning lets us have cheap copies and lifetime upgrades for small slices (up to 39 bytes on 64bit archs), as copying them no longer requires an allocation if their original storage doesn't support reference counting or lifetime extension.

This PR also:

• Removes From for non-static slices, so that From and from_static are now equivalent, and from_slice is now the constructor from borrowed slices. The reasoning behind this is to make the more wasteful variant less ergonomic
• Adds constructors From &'static [u8;N] and &'static str to reduce boilerplate.

[danielhenrymantilla]

LGTM ✅

• Naming nit: interning is a term usually referred to using some arena or whatnot to "cache" common slices/strings, etc.. What would you think of using inlining instead? (and mention "small vec optimization" in the docs alongside it)

[danielhenrymantilla]

⚠️

[p-avital]

Thanks for catching that endianness issue! Your reviews are always awesome!

I ignored some of the comments that clashed with allowing certain functions to be const :)