Add methods for vector generics

[jonocarroll]

As much as possible, functions taking a vec should return a vec. Use {generics} if possible.

• [ ] c()
• [ ] head(), `tail()
• [ ] reverse()
• [ ] sort()
• [ ] `set-wise operations

[jrosell]

One suggestion, it could be great to be able to set:

as_vec( a, b, )

Just like rlang::list2 does using ... param.