Add an extension function asAddress(self: Slice) to the stdlib

[Shvandre]

Idk, maybe slice should be checked if it is a valid address
Or it may be just "NOP"

[novusnota]

Perhaps, we should add two: with and without a check. Like, .asAddress() won't check, but .asValidAddress() (or something) will. That's because we've previously removed lots of Address checks, but having some that user can tap in if they want seems beneficial to me.

Ideas of names for the latter:

• .asAddressCheck()
• .asAddressAssert() (akin to !! in name and spirit) — or even .assertAddress()
• .asAddressSafe()

[Shvandre]

Great idea!
Maybe it's better to make .asAddress safe version, and .asAddressUnsafe - without any checks.