Migrate content length header & co

[phongngtuan]

Following up the session led by @rossabaker, partially address #4535

• Add syntax for Header & Select
• Add implicit from Select[A] to Renderer[A]
• Migrate some of headers

Only the first commit is more or less interesting where the machinery are introduced. Really appreciate if you could take a look before I stray too far from the path. In particular

• SimpleHeadersSpec used to take key + value but now only works on value.
• HeadersSpec

todo:

• migrate the rest of the headers
• revisit the headers classes to clean up unused method/val (key, name,...) that was from inheritance. I left those as is since I don't know if anyone is using those

[rossabaker]

I think this is the only case where an http4s model type appears in this file. If it works putting this on Select instead of Renderer, I think that would be preferrable. I also just want to get this all compiling again, so I'm content to revisit this later.