build: drop more forward function declarations

Most by moving functions around. Also delete unused ones.
Reducing their number from 83 to 33.

Remaining ones due to:
- circular dependencies.
- H3 code, that I did not attempt to update and likely the above applies.
- static declarations with attributes (`CURL_PRINTF`, `WARN_UNUSED_RESULT`).
- OS400 code.

Closes #20321

lib/cw-out.c

@@ -101,22 +101,6 @@ struct cw_out_ctx {
   BIT(errored);
 };
 
-static CURLcode cw_out_write(struct Curl_easy *data,
-                             struct Curl_cwriter *writer, int type,
-                             const char *buf, size_t nbytes);
-static void cw_out_close(struct Curl_easy *data, struct Curl_cwriter *writer);
-static CURLcode cw_out_init(struct Curl_easy *data,
-                            struct Curl_cwriter *writer);
-
-const struct Curl_cwtype Curl_cwt_out = {
-  "cw-out",
-  NULL,
-  cw_out_init,
-  cw_out_write,
-  cw_out_close,
-  sizeof(struct cw_out_ctx)
-};
-
 static CURLcode cw_out_init(struct Curl_easy *data,
                             struct Curl_cwriter *writer)
 {
@@ -455,6 +439,15 @@ static CURLcode cw_out_write(struct Curl_easy *data,
   return CURLE_OK;
 }
 
+const struct Curl_cwtype Curl_cwt_out = {
+  "cw-out",
+  NULL,
+  cw_out_init,
+  cw_out_write,
+  cw_out_close,
+  sizeof(struct cw_out_ctx)
+};
+
 bool Curl_cw_out_is_paused(struct Curl_easy *data)
 {
   struct Curl_cwriter *cw_out;