Create CONVERT opcode about neo-vm

[shargon]

At the end, the others OpCodes make the conversion, so i don't get the point 😅

For example: When MUL receive a Bool or ByteArray, it's converted to BigInteger

[igormcoelho]

Man, I gave you 8 situations where no conversion exists xD

[igormcoelho]

Any thoughts @dicarlo2 @shargon ?

[lightszero]

still need compiler，so what c# code use convert opcode？ show some case maybe better.

[igormcoelho]

I think that this can be done together with the CLONE idea @lightszero ... perhaps a CloneAs that chooses the selected type after conversion.