Deprecate TrieSet.mem() in favor of TrieSet.has() (#576)

This PR deprecates `TrieSet.mem()` in favor of `TrieSet.has()` for
consistency with the [base library design
document](https://github.com/dfinity/motoko-base/blob/master/doc/design.md#containers).

We could update the design guide if this makes more sense, although I
think `has` would be less ambiguous than `mem`, which could be
misunderstood as having something to do with "memory" instead of
"membership" for developers familiar with wording such as "has" /
"contains" / "includes".

src/TrieSet.mo

@@ -104,8 +104,15 @@ module {
     return true;
   };
 
+  /// @deprecated: use `TrieSet.contains()`
+  ///
   /// Test if a set contains a given element.
   public func mem<T>(s : Set<T>, x : T, xh : Hash, eq : (T, T) -> Bool) : Bool {
+    contains(s, x, xh, eq)
+  };
+
+  /// Test if a set contains a given element.
+  public func contains<T>(s : Set<T>, x : T, xh : Hash, eq : (T, T) -> Bool) : Bool {
     switch (Trie.find<T, ()>(s, { key = x; hash = xh }, eq)) {
       case null { false };
       case (?_) { true }

test/TrieSet.test.mo

@@ -20,6 +20,11 @@ let simpleTests = do {
         TrieSet.mem<Nat>(set1, 1, 1, Nat.equal),
         M.equals(T.bool true)
       ),
+      Suite.test(
+        "contains",
+        TrieSet.contains<Nat>(set1, 1, 1, Nat.equal),
+        M.equals(T.bool true)
+      ),
       Suite.test(
         "size",
         TrieSet.size(set1),