Make HeaderVec more Vec like

[cehteh]

Whats your opinion about breaking changes that make HeaderVec more Vec like? With the long-time goal to be mostly a drop in replacement to std::Vec where it makes sense. Of course it needs to retain its unique API as it has now (access to the header, HeaderVecWeak, the atomic API I added)

notably change/additions:

• Deref should result in &[T] not the header. provide a public .header() method to get the user header (the internal header() has to be renamed )
• reserve all method names of std::Vec to be implemented opportunistically, there are a lot, many are somewhat uncommon and don't need to be implemented ASAP. .get() is most importantly missing, many others are also prolly only oneliners, like .iter() etc
• add a "stdlib" feature, then methods that require std things can be conditionally included
• implement (most of) the Traits that std::Vec implements for HeaderVec as well (possibly guarded by the stdlib feature)