Fix SemiRegular.

Fixes #1620.

Another pass to loosen some constraints now that SemiRegular refines
Copyable is warranted.

Author: dabrahams

StandardLibrary/Sources/Array.hylo

@@ -207,7 +207,7 @@ public type Array<Element: SemiRegular>: SemiRegular {
 
 }
 
-public conformance Array: Equatable {
+public conformance Array: Equatable where Element: Equatable {
 
   /// Returns `true` iff `other` has an equivalent value.
   public fun infix== (_ other: Self) -> Bool {

StandardLibrary/Sources/Core/Regular.hylo

@@ -3,9 +3,9 @@
 ///
 /// - Move is value-preserving.
 /// - Destruction has no side-effects.
-public trait SemiRegular: Deinitializable, Movable, Equatable {}
+public trait SemiRegular: Copyable, Deinitializable {}
 
 /// Regular types (roughly per Stepanov).
 ///
 /// Copies have equal value.
-public trait Regular: SemiRegular, Copyable {}
+public trait Regular: SemiRegular, Equatable {}