Put ArchiveCopy behind a feature

[djkoloski]

A major problem with ArchiveCopy types right now is that they may copy uninitialized bytes in their padding to the output. This behavior should be an opt-in feature for performance. There are still some types that can always be safely copied, so the trait needs to be split up.

• ArchiveCopy needs to be split into ArchiveCopy (auto trait, explicit impl until auto trait is stabilized, this can be copied but may have padding and produce uninitialized memory) and ArchiveCopySafe (explicit impl, this is ArchiveCopy and has no padding).
• All types implement Archive the exact same way whether they're ArchiveCopy or not. This includes performing field-by-field resolving even for types that are ArchiveCopy.
• Types that are ArchiveCopy may implement the traits and no more.
• Optimizations may always take advantage of ArchiveCopySafe types. They may also take advantage of ArchiveCopy types, but only when a feature (copy_unsafe?) is enabled.
• Derives produce ArchiveCopySafe impls when passed #[archive(copy_safe)].

[djkoloski]

Core rewrite done in f438fa3

[djkoloski]

copy_unsafe added in a753588

[djkoloski]

#[archive(copy_safe)] added in b17f121