Remove feature flag from post-stabilization feature gates

This commit removes the optional `feature` specification from feature
gates (`@since`, in particular).

This change should simplify the usage of feature gates (see
WebAssembly/component-model#387) for more
discussion.

This is a breaking change for those who were depending on `wit-parser`
as `feature` now no longer present.

Signed-off-by: Victor Adossi <vadossi@cosmonic.com>

crates/wit-parser/src/ast.rs

@@ -1548,7 +1548,6 @@ enum Attribute<'a> {
     Since {
         span: Span,
         version: Version,
-        feature: Option<Id<'a>>,
     },
     Unstable {
         span: Span,
@@ -1571,18 +1570,10 @@ impl<'a> Attribute<'a> {
                     eat_id(tokens, "version")?;
                     tokens.expect(Token::Equals)?;
                     let (_span, version) = parse_version(tokens)?;
-                    let feature = if tokens.eat(Token::Comma)? {
-                        eat_id(tokens, "feature")?;
-                        tokens.expect(Token::Equals)?;
-                        Some(parse_id(tokens)?)
-                    } else {
-                        None
-                    };
                     tokens.expect(Token::RightParen)?;
                     Attribute::Since {
                         span: id.span,
                         version,
-                        feature,
                     }
                 }
                 "unstable" => {

crates/wit-parser/src/ast/resolve.rs

@@ -1386,28 +1386,20 @@ impl<'a> Resolver<'a> {
         match attrs {
             [] => Ok(Stability::Unknown),
 
-            [ast::Attribute::Since {
-                version, feature, ..
-            }] => Ok(Stability::Stable {
+            [ast::Attribute::Since { version, .. }] => Ok(Stability::Stable {
                 since: version.clone(),
-                feature: feature.as_ref().map(|s| s.name.to_string()),
                 deprecated: None,
             }),
 
-            [ast::Attribute::Since {
-                version, feature, ..
-            }, ast::Attribute::Deprecated {
+            [ast::Attribute::Since { version, .. }, ast::Attribute::Deprecated {
                 version: deprecated,
                 ..
             }]
             | [ast::Attribute::Deprecated {
                 version: deprecated,
                 ..
-            }, ast::Attribute::Since {
-                version, feature, ..
-            }] => Ok(Stability::Stable {
+            }, ast::Attribute::Since { version, .. }] => Ok(Stability::Stable {
                 since: version.clone(),
-                feature: feature.as_ref().map(|s| s.name.to_string()),
                 deprecated: Some(deprecated.clone()),
             }),
 

crates/wit-parser/src/lib.rs

@@ -845,8 +845,6 @@ pub enum Stability {
         #[cfg_attr(feature = "serde", serde(serialize_with = "serialize_version"))]
         #[cfg_attr(feature = "serde", serde(deserialize_with = "deserialize_version"))]
         since: Version,
-        #[cfg_attr(feature = "serde", serde(skip_serializing_if = "Option::is_none"))]
-        feature: Option<String>,
         #[cfg_attr(
             feature = "serde",
             serde(