Update & simplify message data model

[eemeli]

This builds on #38, and will need a rebase after that's merged. Actual diff here is reorg-paths...simpler-messages.

This applies the following upstream changes made to the MF2 data model:

• Accept attributes design & remove spec note unicode-org/message-format-wg#845
  • Do not allow variable references as attribute values
• Match on variables instead of expressions unicode-org/message-format-wg#877
  • Use variables rather than expressions as selectors
• Remove forward-compatibility promise and all reserved & private syntax unicode-org/message-format-wg#883
  • "unsupported" constructs are dropped
  • Expression.annotation is renamed as Expression.function

Going further from there, the message data model is also made more ergonomic:

• Declarations are required for SelectMessage, as at least one is needed for the selector.
• Declarations are defined by a dict, rather than a list of (name, expression) tuples.
• SelectMessage.selectors is a tuple, not a list.
• FunctionAnnotation is flattened into Expression.function and Expression.options.

As far as I can tell, no external users should be impacted by these changes.

[mathjazz]

I didn't run the code, but nothing in the changes and tests stands our. r+