int8recv and substring(bytea) for correctness checking

[lmatz]

In PG, we use the following:

SELECT BIT_XOR(('x'||SUBSTR(MD5(%s), 17, 16))::bit(64)::bigint) FROM %s

to compare the results of two queries.

However, since RW hasn't supported the bit data type, we are unable to convert the hexadecimal varchar into a bigint.

@xiangjinwu gives the solution that:

1. we need a int8recv(bytea) -> bigint, which is already supported as an internal function i64::from_be_bytes. Expose it to user.
2. we need a substring(bytea).

Once we have these two supported, we write the following instead:

int8recv(SUBSTR(decode(MD5(%s), 'hex'), 9, 8))
int8recv(SUBSTR(sha256(%s), 9, 8))

cc: @cyliu0