change semantics of @memcpy and @memset

Now they use slices or array pointers with any element type instead of
requiring byte pointers.

This is a breaking enhancement to the language.

The safety check for overlapping pointers will be implemented in a
future commit.

closes #14040

test/behavior/basic.zig

@@ -367,8 +367,8 @@ fn testMemcpyMemset() !void {
     var foo: [20]u8 = undefined;
     var bar: [20]u8 = undefined;
 
-    @memset(&foo, 'A', foo.len);
-    @memcpy(&bar, &foo, bar.len);
+    @memset(&foo, 'A');
+    @memcpy(&bar, &foo);
 
     try expect(bar[0] == 'A');
     try expect(bar[11] == 'A');