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 {}

Tests/HyloTests/TestCases/TypeChecking/AbstractType.hylo

@@ -1,14 +1,19 @@
 //- typeCheck expecting: .success
 
-trait P: SemiRegular {
-  type T: SemiRegular
-  type U: SemiRegular
+// Our SemiRegular when this test was written wasn't the genuine
+// SemiRegular but was being depended on by this test, so rephrasing
+// it here.
+trait LegacySemiRegular: Deinitializable, Movable, Equatable {}
+
+trait P: LegacySemiRegular {
+  type T: LegacySemiRegular
+  type U: LegacySemiRegular
 
   fun t(_ x: T)
   fun u(_ x: U)
 }
 
-trait Q: SemiRegular {
+trait Q: LegacySemiRegular {
   type V: P
   property v: V { let }
 }