feat: drop Diff in favor of Exclude

We don't need `Diff` any longer since `Exclude` exists (and has for a
while). `Diff` is just a wrapper over `Exclude` now anyways.

BREAKING CHANGE: `Diff` is no longer exported from simplytyped. Can find
and replace `Diff` for `Exclude` to maintain same functionality.

src/types/objects.ts

@@ -1,4 +1,3 @@
-import { Diff } from './strings';
 import { False, True } from './conditionals';
 import { AnyFunc } from './functions';
 
@@ -127,7 +126,7 @@ export type UnionizeProperties<T extends object> = T[Keys<T>];
  * @param K the union of keys to remove from `T`
  * @returns `T` with the keys `K` removed
  */
-export type Omit<T extends object, K extends Keys<T>> = Pick<T, Diff<Keys<T>, K>>;
+export type Omit<T extends object, K extends Keys<T>> = Pick<T, Exclude<Keys<T>, K>>;
 /**
  * Returns only the shared properties between two objects.
  * All shared properties must be the same type.

src/types/strings.ts

@@ -1,15 +1,12 @@
 import { True, False, And } from './conditionals';
 import { IsNever } from './predicates';
 
-// TODO: this is here only for backwards compatibility
-export type Diff<T, U> = Exclude<T, U>;
-
-export type DropString<T extends string, U extends T> = Diff<T, U>;
+export type DropString<T extends string, U extends T> = Exclude<T, U>;
 
 export type StringEqual<T extends string, U extends string> =
     And<
-        IsNever<Diff<T, U>>,
-        IsNever<Diff<U, T>>
+        IsNever<Exclude<T, U>>,
+        IsNever<Exclude<U, T>>
     >;
 
 export type UnionContains<T, U> = U extends T ? True : False;