refactor(headers): Use header!() for CORS headers.

This is the last bunch of headers that should use the new macro. Moved them out of
their own folder so that the macro works. Changed them, so that they are more in
line with the other headers.

BREAKING CHANGE: `AccessControlAllowHeaders` and `AccessControlRequestHeaders` values
are case insensitive now. `AccessControlAllowOrigin` variants are now `Any` and
`Value` to match the other headers.

src/header/common/access_control_max_age.rs

@@ -0,0 +1,8 @@
+header! {
+    #[doc="`Access-Control-Max-Age` header, part of"]
+    #[doc="[CORS](www.w3.org/TR/cors/#access-control-max-age-response-header)"]
+    #[doc=""]
+    #[doc="The `Access-Control-Max-Age` header indicates how long the results of a"]
+    #[doc="preflight request can be cached in a preflight result cache."]
+    (AccessControlMaxAge, "Access-Control-Max-Age") => [u32]
+}