examples: Extend sig examples by call that uses static context

Besides improving the examples, this makes sure that the examples
import a variable (instead of a function), namely the static context,
from the library. This is helpful when testing MSVC builds, because
the MSVC linker tends to be awkward when importing variables.

Author: real-or-random

examples/ecdsa.c

@@ -34,7 +34,7 @@ int main(void) {
     unsigned char compressed_pubkey[33];
     unsigned char serialized_signature[64];
     size_t len;
-    int is_signature_valid;
+    int is_signature_valid, is_signature_valid2;
     int return_val;
     secp256k1_pubkey pubkey;
     secp256k1_ecdsa_signature sig;
@@ -116,10 +116,18 @@ int main(void) {
     printf("Signature: ");
     print_hex(serialized_signature, sizeof(serialized_signature));
 
-
     /* This will clear everything from the context and free the memory */
     secp256k1_context_destroy(ctx);
 
+    /* Bonus example: if all we need is signature verification (and no key
+       generation or signing), we don't need to use a context created via
+       secp256k1_context_create(). We can simply use the static (i.e., global)
+       context secp256k1_context_static. See its description in
+       include/secp256k1.h for details. */
+    is_signature_valid2 = secp256k1_ecdsa_verify(secp256k1_context_static,
+                                                 &sig, msg_hash, &pubkey);
+    assert(is_signature_valid2 == is_signature_valid);
+
     /* It's best practice to try to clear secrets from memory after using them.
      * This is done because some bugs can allow an attacker to leak memory, for
      * example through "out of bounds" array access (see Heartbleed), Or the OS

examples/schnorr.c

@@ -26,7 +26,7 @@ int main(void) {
     unsigned char auxiliary_rand[32];
     unsigned char serialized_pubkey[32];
     unsigned char signature[64];
-    int is_signature_valid;
+    int is_signature_valid, is_signature_valid2;
     int return_val;
     secp256k1_xonly_pubkey pubkey;
     secp256k1_keypair keypair;
@@ -135,6 +135,15 @@ int main(void) {
     /* This will clear everything from the context and free the memory */
     secp256k1_context_destroy(ctx);
 
+    /* Bonus example: if all we need is signature verification (and no key
+       generation or signing), we don't need to use a context created via
+       secp256k1_context_create(). We can simply use the static (i.e., global)
+       context secp256k1_context_static. See its description in
+       include/secp256k1.h for details. */
+    is_signature_valid2 = secp256k1_schnorrsig_verify(secp256k1_context_static,
+                                                      signature, msg_hash, 32, &pubkey);
+    assert(is_signature_valid2 == is_signature_valid);
+
     /* It's best practice to try to clear secrets from memory after using them.
      * This is done because some bugs can allow an attacker to leak memory, for
      * example through "out of bounds" array access (see Heartbleed), Or the OS