crypto: replace non-standard CLZ builtins with c++20's bit_width

Also some header cleanups.

src/crypto/common.h

@@ -5,15 +5,12 @@
 #ifndef BITCOIN_CRYPTO_COMMON_H
 #define BITCOIN_CRYPTO_COMMON_H
 
-#if defined(HAVE_CONFIG_H)
-#include <config/bitcoin-config.h>
-#endif
-
-#include <stdint.h>
-#include <string.h>
-
 #include <compat/endian.h>
 
+#include <bit>
+#include <cstdint>
+#include <cstring>
+
 uint16_t static inline ReadLE16(const unsigned char* ptr)
 {
     uint16_t x;
@@ -89,22 +86,7 @@ void static inline WriteBE64(unsigned char* ptr, uint64_t x)
 /** Return the smallest number n such that (x >> n) == 0 (or 64 if the highest bit in x is set. */
 uint64_t static inline CountBits(uint64_t x)
 {
-#if HAVE_BUILTIN_CLZL
+    return std::bit_width(x);
-    if (sizeof(unsigned long) >= sizeof(uint64_t)) {
-        return x ? 8 * sizeof(unsigned long) - __builtin_clzl(x) : 0;
-    }
-#endif
-#if HAVE_BUILTIN_CLZLL
-    if (sizeof(unsigned long long) >= sizeof(uint64_t)) {
-        return x ? 8 * sizeof(unsigned long long) - __builtin_clzll(x) : 0;
-    }
-#endif
-    int ret = 0;
-    while (x) {
-        x >>= 1;
-        ++ret;
-    }
-    return ret;
 }
 
 #endif // BITCOIN_CRYPTO_COMMON_H