Simplifying FromMeta trait

[TedDriggs]

Currently, FromMeta has a number of different optional methods, which correspond to different values that might be encountered when parsing. With the release of syn 2.0, does it make sense to streamline the trait down to only three methods:

trait FromMeta {
    fn from_meta(meta: &Meta) -> Result<Self>;
    /// We'd still recommend not inspecting the value of `path` here, but passing it through
    /// gives the implementer access to the span and the value for generating diagnostics.
    fn from_path(path: &Path) -> Result<Self>;
    /// Not sure what type this should take; the current slice has its problems, but I do
    /// like that the slice provides a way to filter out or inject list items into the collection
    fn from_list(??) -> Result<Self>;
    fn from_expr(expr: &Expr) -> Result<Self>;
    /// This will continue to be used to fill in optional fields.
    fn from_none() -> Option<Self>;
}

darling could provide utility functions or Ext traits to make custom impls of from_expr easier; their exact design would need further investigation.

Advantages

Consolidating these methods would make FromMeta much simpler: Each variant of syn::Meta would correspond to one of the overridable functions, with a total takeover being possible-but-not-recommended using from_meta

This consolidation also ensures that each function has access to the best available span information, which is important for building good diagnostic messages.

Disadvantages

1. I don't know how many users of the crate this would break.
2. Without good helpers, writing a custom FromMeta that wants to do something simple (like parse a string) will become more verbose, and the barrier to customization will go up. It should be dead-simple to write a custom impl for a newtype struct that wraps an IpV4Addr, for example.