Support as operator and explicit signatures

[MaxDesiatov]

The simplest use case is support for Any subtyping, but we also need to support existential types in the future, such as

protocol P {}
extension Int: P {}
let x = 5 as P

After handling of as is implemented, we would be able to handle explicit signatures properly:

let x: P = 5

by internally converting this declaration to AST that's equivalent to let x = 5 as P.