refactor header helpers

[cataggar]

 pub fn get_as_str<T: Into<HeaderName>>(&self, key: T) -> Option<&str> {
        self.get(key).map(|v| v.as_str())
    }

I think we should just get rid of the variants of this methods and only have the variants which return a crate::Result. If you need an option, you can just call .ok() on the Result.
And of course, then we just use the simpler naming convention and not the or_err names.
#833 (review)

    pub fn get_as_enum<T: Into<HeaderName>, V: FromStr<Err = E>, E>(

This is a bit more general than for enums. Can we call this get_as_parsed or something like that? I think get_as_u64_or_err can be implemented using this function.

[cataggar]

get_as_parsed sounds good. I'm not sure about removing the variants. Right now:

• Option is used if the header is optional
• or_err variants are a shortcut for unwrap_or_err for when the header is required, if return a Result::Err.
• the get_as_u64 and get_as_enum may fail parsing, and thus return a Result::Err

Some options I'd like to explore:

• keep fallible conversions separate and remove get_as_u64 and get_as_enum
• renaming or_err functions
  • may be require, require_str and require_string

[bmc-msft]

I believe this was addressed by #857

[rylev]

The last thing to consider is renaming some of the methods. I'll take a look at that tomorrow.

[rylev]

This was finished in #864