Expose info of mixins targeting a class and info from a mixin class

Allow external code to query the list of mixins that target a class,
including mixins that were not applied, by exposing getMixins; this
method wasn't used internally but exposing it allows external error
handlers greater visibility of mixins targeting a class.

ClassInfo.mixin is also exposed with getMixinInfo to allow external
code to retrieve mixin metadata from a class. This supersedes the need
for some other methods like isMixin and isLoadable, but these are kept
for compatibility.

src/main/java/org/spongepowered/asm/mixin/transformer/ClassInfo.java

@@ -938,8 +938,8 @@ void addAppliedMixin(MixinInfo mixin) {
     /**
      * Get all mixins which target this class
      */
-    Set<MixinInfo> getMixins() {
-        return this.isMixin ? Collections.<MixinInfo>emptySet() : Collections.<MixinInfo>unmodifiableSet(this.mixins);
+    public Set<IMixinInfo> getMixins() {
+        return this.isMixin ? Collections.<IMixinInfo>emptySet() : Collections.<IMixinInfo>unmodifiableSet(this.mixins);
     }
 
     /**
@@ -955,6 +955,13 @@ public Set<IMixinInfo> getAppliedMixins() {
     public boolean isMixin() {
         return this.isMixin;
     }
+
+    /**
+     * Get mixin metadata for this class, if it is a mixin
+     */
+    public IMixinInfo getMixinInfo() {
+        return this.mixin;
+    }
 
     /**
      * Get whether this class is loadable mixin