There is no point in encoding the output of this function in a WIF format,
most of the users actually want the real key and those who need a WIF can
easily get if from the key (and it's simpler than getting the key from the
WIF).
It also fixes a severe bug in NEP2Decrypt, base58 decoding errors were not
processed correctly.
It reduces heap pressure a little for these elements as we don't have to
allocate/free them individually. And they're directly tied to transactions or
block, not being shared or anything like that, so it makes little sense for
them to be pointer-based. It only makes building transactions a little easier,
but that's obviously a minor usecase.
This allows easier reuse of opcodes and in some cases allows to eliminate
dependencies on the whole vm package, like in compiler that only needs opcodes
and doesn't care about VM for any other purpose.
And yes, they're opcodes because an instruction is a whole thing with
operands, that's what context.Next() returns.
