Support for ResizeSector and ProposeUpload

Implement opcodes for `ResizeSectorErase()` and `ProposeUpload()`. Add script generator functions for those opcodes. Add client command for `ResizeSectorErase()`.

Due to the large number of inputs to `ProposeUpload()`, a gob-encodable struct has been created to hold its arguments. Much like `AddSibling()`, the relevant arguments must be encoded as a byte slice before they can be processed by the opcode.