include/encoding: add encoder helpers for sized ints

include/encoding: add encoder helpers for sized ints

When the raw type may not match the required encoded size, this helper makes
intent clear and avoids a common verbose pattern:

    intX_t t = val;
    encode(t, bl);

and

    intX_t t;
    decode(t, p);
    val = t;

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>