Macro from_syn_parse 

macro_rules! from_syn_parse {
    ($ty:path) => { ... };
}

Adapter from syn::parse::Parse to FromMeta for items that cannot be expressed in a syn::MetaNameValue.

This cannot be a blanket impl, due to the syn::Lit family’s need to handle non-string values. Therefore, we use a macro and a lot of impls.