Sema: @memcpy changes

* The langspec definition of `@memcpy` has been changed so that the
  source and destination element types must be in-memory coercible,
  allowing all such calls to be raw copying operations, not actually
  applying any coercions.
* Implement aliasing check for comptime `@memcpy`; a compile error will
  now be emitted if the arguments alias.
* Implement more efficient comptime `@memcpy` by loading and storing a
  whole array at once, similar to how `@memset` is implemented.

test/cases/compile_errors/memcpy_alias.zig

@@ -0,0 +1,14 @@
+var arr: [10]u64 = undefined;
+export fn foo() void {
+    @memcpy(arr[0..6], arr[4..10]);
+}
+
+comptime {
+    var types: [4]type = .{ u8, u16, u32, u64 };
+    @memcpy(types[2..4], types[1..3]);
+}
+
+// error
+//
+// :3:5: error: '@memcpy' arguments alias
+// :8:5: error: '@memcpy' arguments alias

test/cases/compile_errors/memcpy_bad_type.zig

@@ -0,0 +1,10 @@
+const src: [10]u8 = @splat(0);
+var dest: [10]u16 = undefined;
+
+export fn foo() void {
+    @memcpy(&dest, &src);
+}
+
+// error
+//
+// :5:5: error: pointer element type 'u8' cannot coerce into element type 'u16'