Use conventional meta-item syntax for attributes

Using the conventional form for attributes, which is compatible with
rustfmt and syn's meta-item parsing. This change is breaking and
substantial, but the necessary edits are largely mechanical.

With meta-items, it becomes possible to offload attribute parsing and
validation to `darling`. This offload fixes rustwasm#2874, as `darling` will
automatically generate errors for unexpected meta-item names. This also
makes it easier for people who are new to the codebase to understand
how to add new options.

This commit does not build: It changes the macro parser, but does not update the usage of the macro.

Author: Ted Driggs

crates/backend/Cargo.toml

@@ -17,6 +17,7 @@ extra-traits = ["syn/extra-traits"]
 
 [dependencies]
 bumpalo = "3.0.0"
+darling = "0.14.1"
 lazy_static = "1.0.2"
 log = "0.4"
 proc-macro2 = "1.0"

crates/backend/src/error.rs

@@ -98,6 +98,12 @@ impl From<Error> for Diagnostic {
     }
 }
 
+impl From<darling::Error> for Diagnostic {
+    fn from(err: darling::Error) -> Self {
+        Self::from(Error::from(err))
+    }
+}
+
 fn extract_spans(node: &dyn ToTokens) -> Option<(Span, Span)> {
     let mut t = TokenStream::new();
     node.to_tokens(&mut t);

crates/macro-support/Cargo.toml

@@ -17,6 +17,7 @@ extra-traits = ["syn/extra-traits"]
 strict-macro = []
 
 [dependencies]
+darling = "0.14.1"
 syn = { version = '1.0.67', features = ['visit', 'full'] }
 quote = '1.0'
 proc-macro2 = "1.0"