Initial code.

src/FORMAT

+scrypt encrypted data format
+----------------------------
+
+offset  length
+0       6       "scrypt"
+6       1       scrypt data file version number (== 0)
+7       1       log2(N) (must be between 1 and 63 inclusive)
+8       4       r (big-endian integer; must satisfy r * p < 2^30)
+12      4       p (big-endian integer; must satisfy r * p < 2^30)
+16      32      salt
+48      16      first 16 bytes of SHA256(bytes 0 .. 47)
+64      32      HMAC-SHA256(bytes 0 .. 63)
+96      X       data xor AES256-CTR key stream generated with nonce == 0
+96+X    32      HMAC-SHA256(bytes 96 .. 96 + (X - 1))
+
+AES256-CTR is computed with a 256-bit AES key key_enc, and HMAC-SHA256 is
+computed with a 256-bit key key_hmac, where
+  scrypt(password, salt, N, r, p, 64) == [key_enc][key_hmac]