refactor: Take Span in SetSeed

This makes calling code less verbose and less fragile. Also, by adding
the CKey::data() member function, it is now possible to call HexStr()
with a CKey object.

src/key.h

@@ -85,6 +85,7 @@ public:
 
     //! Simple read-only vector-like interface.
     unsigned int size() const { return (fValid ? keydata.size() : 0); }
+    const unsigned char* data() const { return keydata.data(); }
     const unsigned char* begin() const { return keydata.data(); }
     const unsigned char* end() const { return keydata.data() + size(); }
 
@@ -177,7 +178,7 @@ struct CExtKey {
     void Decode(const unsigned char code[BIP32_EXTKEY_SIZE]);
     bool Derive(CExtKey& out, unsigned int nChild) const;
     CExtPubKey Neuter() const;
-    void SetSeed(const unsigned char* seed, unsigned int nSeedLen);
+    void SetSeed(Span<const uint8_t> seed);
 };
 
 /** Initialize the elliptic curve support. May not be called twice without calling ECC_Stop first. */