Updates following the first round of review comments. (#2313)

Added more `Codable` and `Sendable` conformances.

Changed `Address`, `Image` and `Symbol` to be `struct`s.

Renamed `Symbol` to `SymbolInfo`, since otherwise `offset` makes little
sense.

Don't expose `FrameSequence`; just have `frames` return `some Sequence<Frame>`.

Change the `uuid` and `buildID` members to `imageID`.

Expose the no-cache flag (from dyld) as `hasSharedCache` instead.

Use an `OptionSet` for options for the `symbolicated` method, and add
a `showSourceLocations` option.

Updated various comments, as well as the discussion about other considerations.

Author: al45tair