net/http: remove badStringError, make some unexported structs non-comparable

net/http: remove badStringError, make some unexported structs non-comparable

Reduces binary size by 4K, not counting the http2 changes (in CL
231119) that'll be bundled into this package in the future.

Updates golang/go#38782

Change-Id: Id360348707e076b8310a8f409e412d68dd2394b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/231118
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>