rework toad-msg module structure

[cakekindel]

toad-msg could use a little white-gloving WRT the module and file structure, it's gotten a bit messy.

public api:

- struct Message
- struct Code
- struct Token
- struct Id
- struct Ver
- struct Type
- struct Opt
- struct OptIter
- struct OptRef
- struct OptRefIter
- enum SetError (prev. SetOptionError)
- enum MessageFromBytesError (prev. MessageParseError)
- enum OptionFromBytesError (prev. OptParseError)
- trait KnownOptions (prev. MessageOptions)
- trait TryIntoBytes
- trait TryFromBytes
- mod opt
  - struct Num (prev. OptNumber)
  - struct Val (prev. OptValue)
  - enum Importance
    - Critical
    - Elective
  - enum CacheKey
    - Include
    - Omit
  - enum ForwardSafety
    - Safe
    - Unsafe
  - enum ContentFormat
  - enum ObserveAction
  - trait Map (prev. OptionMap)
  - const ACCEPT, CONTENT_FORMAT, HOST...

private:

- mod msg
  - struct Message
  - struct Code
  - struct Token
  - struct Id
  - struct Ver
  - struct Type
  - enum SetError
- mod bytes
  - enum MessageFromBytesError (prev. MessageParseError)
  - enum OptionFromBytesError (prev. OptParseError)
  - trait TryIntoBytes
  - trait TryFromBytes
- mod opt
  - struct Opt
  - struct OptIter
  - struct OptRef
  - struct OptRefIter
  - enum Importance
    - Critical
    - Elective
  - enum CacheKey
    - Include
    - Omit
  - enum ForwardSafety
    - Safe
    - Unsafe
  - trait KnownOptions (prev. MessageOptions)
  - trait Map (prev. OptionMap)
  - mod num
    - struct Num (prev. OptNumber)
    - const ACCEPT, CONTENT_FORMAT, HOST...
  - mod val
    - struct Val (prev. OptValue)
    - enum ContentFormat
    - enum ObserveAction

// in toad-msg/src/lib.rs
#[doc(inline)] pub use msg::{Message, Code, Token, Id, Ver, Type, SetError};
#[doc(inline)] pub use opt::{Opt, OptRef, OptIter, OptRefIter, KnownOptions};
#[doc(inline)] pub use bytes::{TryFromBytes, TryIntoBytes, MessageFromBytesError, OptionFromBytesError};

// in toad-msg/src/opt/mod.rs
#[doc(inline)] pub use num::{Num, ACCEPT, CONTENT_FORMAT, ...};
#[doc(inline)] pub use val::{Val, ContentFormat, ObserveAction};