vm: Buffer to Integer conversion inconsistency

[roman-khimov]

Buffer type doesn't implement GetInteger() (unlike GetBoolean()) and therefore arithmetic/bitwise opcodes can't take it for for input. Yet at the same time it has a ConvertTo() implementation that allows to CONVERT it to StackItemType.Integer. This seems to be somewhat inconsistent, I'd say it should have a GetInteger() implementation.

Related to #493 (similar conversion optimization could be done for integers if not this behavior).

[shargon]

For me is good to allow GetInteger

[shargon]

We can do it, but we don't have access to hardforks in vm types

[erikzhang]

GetInteger() performs an implicit conversion, while ConvertTo() performs an explicit conversion. It's normal for them to be different.