[PATCH] Add get_bytes() and set_bytes() functions

Hi,

While discussing another patch [1] it was discovered that we don't
have a convenient way of casting a bytea to an integer / bigint and
vice versa, extracting integers larger than one byte from byteas, etc.

For instance, casting '\x11223344' to 0x11223344 may look like this:

```
WITH vals AS (
      SELECT '\x11223344'::bytea AS x
)
SELECT
    (get_byte(x, 0) :: bigint << 24) |
    (get_byte(x, 1) << 16) |
    (get_byte(x, 2) << 8) |
    get_byte(x, 3)
FROM vals;
```

There seems to be a demand for this functionality [2][3] and it costs
us nothing to maintain it, so I propose adding it.

The proposed patch adds get_bytes() and set_bytes() functions. The
semantics is similar to get_byte() and set_byte() we already have but
the functions operate with bigints rather than bytes and the user can
specify the size of the integer. This allows working with int2s,
int4s, int8s or even int5s if needed.

Examples:

```
SELECT get_bytes('\x1122334455667788'::bytea, 1, 2) = 0x2233;
 ?column?
----------
 t

SELECT set_bytes('\x1122334455667788'::bytea, 1, 2, 0xAABB);
     set_bytes
--------------------
 \x11aabb4455667788
```

Thoughts?

[1]: https://postgr.es/m/CAJ7c6TNMTGnqnG%3DyXXUQh9E88JDckmR45H2Q%2B%3DucaCLMOW1QQw%40mail.gmail.com
[2]: https://stackoverflow.com/questions/32944267/postgresql-converting-bytea-to-bigint
[3]: https://postgr.es/m/AANLkTikip9xs8iXc8e%2BMgz1T1701i8Xk6QtbVB3KJQzX%40mail.gmail.com
-- 
Best regards,
Aleksander Alekseev