Refactor wasi:http headers' host representation (bytecodealliance#12754)

* Refactor `wasi:http` headers' host representation

This commit is a follow-on/extension of bytecodealliance#12748 and extends the changes
made for WASIp2 headers in bytecodealliance#12652 to the WASIp3 implementation as well.
This is done through a number of refactorings to make the WASIp2 and
WASIp3 implementations more similar in terms of how they represent
headers. Changes here are:

* `FieldMap` now has its own dedicated module at the crate root instead
  of intermingling with other WASIp2 types.
* `FieldMap` is now internally-`Arc`'d and is cheaply clonable.
  `FieldMap` itself now tracks whether it's mutable or immutable (WASI
  semantics) and doesn't need different wrappers in WASIp2 and WASIp3.
* Creation of an immutable `FieldMap` can be done without needing a size
  limit. Flagging a `FieldMap` as mutable, however, requires a size limit.
* `FieldMap::set` was added to be a bit more efficient w.r.t. clones.
* `FieldMapError` is a new error type that covers all of the possible
  error modes of operating with a `FieldMap`. Conversions from this to
  WASIp{2,3} `header-error` types are now implemented as well.
* WASIp2 now flags `header-error` as a trappable-error-type, allowing
  the use of `?` in implementing header functions (like WASIp3).
* Much of WASIp2's header implementation was refactored with `?`, moving
  methods around, shuffling where headers are made vs `FieldMap`, some
  minor idioms, etc.
* WASIp3 no longer uses `MaybeMutable` for headers and instead uses
  `FieldMap` directly.

cc bytecodealliance#12674

* Clippy warnings

Author: alexcrichton

crates/test-programs/src/bin/p2_cli_http_headers.rs

@@ -1,31 +1,71 @@
-fn main() {
-    let fields = wasip2::http::types::Fields::new();
+use test_programs::p3::wasi as wasip3;
 
+fn main() {
     match std::env::args().nth(1).as_deref() {
-        Some("append") => {
+        Some("p2-append") => {
+            let fields = wasip2::http::types::Fields::new();
+            for i in 0.. {
+                if fields.append(&format!("a{i}"), b"a").is_err() {
+                    break;
+                }
+            }
+        }
+        Some("p2-append-empty") => {
+            let fields = wasip2::http::types::Fields::new();
+            for i in 0.. {
+                if fields.append(&format!("a{i}"), b"").is_err() {
+                    break;
+                }
+            }
+        }
+        Some("p2-append-same") => {
+            let fields = wasip2::http::types::Fields::new();
+            loop {
+                if fields.append("a", b"b").is_err() {
+                    break;
+                }
+            }
+        }
+        Some("p2-append-same-empty") => {
+            let fields = wasip2::http::types::Fields::new();
+            loop {
+                if fields.append("a", b"").is_err() {
+                    break;
+                }
+            }
+        }
+        Some("p3-append") => {
+            let fields = wasip3::http::types::Fields::new();
             for i in 0.. {
                 if fields.append(&format!("a{i}"), b"a").is_err() {
                     break;
                 }
             }
         }
-        Some("append-empty") => {
+        Some("p3-append-empty") => {
+            let fields = wasip3::http::types::Fields::new();
             for i in 0.. {
                 if fields.append(&format!("a{i}"), b"").is_err() {
                     break;
                 }
             }
         }
-        Some("append-same") => loop {
-            if fields.append("a", b"b").is_err() {
-                break;
+        Some("p3-append-same") => {
+            let fields = wasip3::http::types::Fields::new();
+            loop {
+                if fields.append("a", b"b").is_err() {
+                    break;
+                }
             }
-        },
-        Some("append-same-empty") => loop {
-            if fields.append("a", b"").is_err() {
-                break;
+        }
+        Some("p3-append-same-empty") => {
+            let fields = wasip3::http::types::Fields::new();
+            loop {
+                if fields.append("a", b"").is_err() {
+                    break;
+                }
             }
-        },
+        }
         other => panic!("unknown test {other:?}"),
     }