build: use more const

Mostly with `char *` types.

Also:
- mime, x509asn1, tool_operate, lib3207: drop redundant casts.
- examples/smooth-gtk-thread: add missing variable declaration.
- reduce variable scopes.
- tests/server: move `data_to_hex()` to its only user: `sws`.

Closes #20489

docs/examples/http2-pushinmemory.c

@@ -93,7 +93,7 @@ static int server_push_callback(CURL *parent,
                                 struct curl_pushheaders *headers,
                                 void *userp)
 {
-  char *headp;
+  const char *headp;
   int *transfers = (int *)userp;
   (void)parent;
   (void)num_headers;