Registers set via async chain of callbacks to unblock codegen

[hinshun]

• Supersedes https://github.com/openllb/hlb/pull/83

BuiltinDecls no longer have access to the Register, but simply just get the current Value and must return (Value, error). This seems to be cleaner, since it doesn't really need to know about the Register.

(Register).Set still exists but only as syntactic sugar.

(Register).SetAsync is used explicitly when the value is only derived from a callback.

Should speed up any build using multiple images, and any future instruction that have asynchronous tasks (like codegen imports).

[hinshun]

Updated and squashed

[aaronlehmann]

Code LGTM; seems like there's a CI issue