Problem:
- `Span` is not `constexpr` aware.
Solution:
- Add `constexpr` support for all parts that do not require
`reinterpret_cast`.
- Modify tests which use the `constexpr` functions.
Two changes were made
1. copy_to() and copy_trimmed_to() now return how many bytes were
copied.
2. The argument was changed to Span<typename RemoveConst<T>::Type>
because the following would not work:
ReadonlyBytes bytes0;
Bytes bytes1;
// Won't work because this calls Span<const u8>::copy_to(Span<u8>)
// but the method was defined as Span<const u8>::copy_to(Span<const u8>)
bytes0.copy_to(bytes1);
I accidently wrote `Span<RemoveConst<T>>` when I meant
`Span<RemoveConst<T>::Type>`.
Changing that wouldn't be enough though, this constructor can only be
defined if T is not const, otherwise it would redefine the copy
constructor. This can be avoided by overloading the cast operator.
