std.http.Client: support HTTP redirects

 * std.http.Status.Class: add a "nonstandard" enum tag. Instead of
   having `class` return an optional value, it can potentially return
   nonstandard.
 * extract out std.http.Client.Connection from std.http.Client.Request
   - this code abstracts over plain/TLS only
   - this is the type that will potentially be stored in a client's LRU
     connection map
 * introduce two-staged HTTP header parsing
   - API users can rely on a heap-allocated buffer with a maximum limit,
     which defaults to 16 KB, or they can provide a static buffer that
     is borrowed by the Request instance.
   - The entire HTTP header is buffered because there are strings in
     there and they must be accessed later, such as with the case of
     HTTP redirects.
   - When buffering the HTTP header, the parser only looks for the
     \r\n\r\n pattern. Further validation is done later.
   - After the full HTTP header is buffered, it is parsed into
     components such as Content-Length and Location.
 * HTTP redirects are handled, with a maximum redirect count option that
   defaults to 3.
   - Connection: close is always used for now; implementing keep-alive
     connections and an LRU connection pool in std.http.Client is a task
     for another day.

see #2007

lib/std/http.zig

@@ -1,5 +1,4 @@
 pub const Client = @import("http/Client.zig");
-pub const Headers = @import("http/Headers.zig");
 
 pub const Version = enum {
     @"HTTP/1.0",
@@ -219,21 +218,22 @@ pub const Status = enum(u10) {
     }
 
     pub const Class = enum {
+        nonstandard,
         informational,
         success,
         redirect,
         client_error,
         server_error,
     };
 
-    pub fn class(self: Status) ?Class {
+    pub fn class(self: Status) Class {
         return switch (@enumToInt(self)) {
             100...199 => .informational,
             200...299 => .success,
             300...399 => .redirect,
             400...499 => .client_error,
             500...599 => .server_error,
-            else => null,
+            else => .nonstandard,
         };
     }
 
@@ -254,5 +254,4 @@ test {
     _ = Client;
     _ = Method;
     _ = Status;
-    _ = Headers;
 }