VM types are used in debugger, while smartcontract ones are used in
manifest. We can't save only one of them, because conversion in either
side is lossy:
1. VM has `Array` and `Struct` but smartcontract only has `Array`.
2. Smartcontract has `Hash160` etc, which are all `ByteString` or
`Buffer` in VM.
And to spice things a bit more, return type in debugger can be `Void`,
which corresponds to no real stackitem type (as it must exist).
