Problems with AddressStd

[IgorKoval]

At the moment AddressStd can be either addr_none$00 or addr_std$10.

See also the address_std on TVM-Solidity.
On TVM-Solidity opcode LDMSGADDR is used to load address_std. The opcode can load an external address. It can cause inconsistent data.

optional(address_std) is encoded as one bit (does it have a value?) and then the address.
Code generated for loading optional(address_std):

	LDI 1
	SWAP
	NULLROTRIFNOT
	PUSHCONT {
		LDMSGADDR
	}
	IF

Code generated for storing optional(address_std):

	ISNULL
	PUSHCONT {
		DROP
		STZERO
	}
	PUSHCONT {
		SWAP
		STONE
		STSLICE
	}
	IFELSE

Proposals:

1. Support new abi version 2.8.0
2. AddressStd can take only [addr_std$10]. On Solidity, we use LDSTDADDR opcode
3. optional(address_std) is serialized as addr_std$10 for null value or addr_std$10. On TVM-Solidity we use just one opcode to encode/decode value: LDOPTSTDADDR/STOPTSTDADDR

Breaking changes:
Old contracts cannot send messages to new ones if:

1. the old contract used [addr_std$10] for address_std in messages
2. the old contract used optional(address_std) in messages