v1.18: support Pubkey::to_bytes as constexpr (backport of #220) (#1107)

sdk: support Pubkey::to_bytes as constexpr (#220)

(cherry picked from commit 9b16df2)

Co-authored-by: ripatel-fd <ripatel+git@jumptrading.com>

sdk/program/src/pubkey.rs

@@ -627,7 +627,7 @@ impl Pubkey {
         }
     }
 
-    pub fn to_bytes(self) -> [u8; 32] {
+    pub const fn to_bytes(self) -> [u8; 32] {
         self.0
     }