Basic operations on MLList

These functions can be upstreamed to Std when convenient.

def MLList.singletonM {m : Type u_1 → Type u_1} {α : Type u_1} [Monad m] (x : m α) : MLList m α

Construct a singleton monadic lazy list from a single monadic value.

def MLList.singleton {m : Type u_1 → Type u_1} {α : Type u_1} [Monad m] (x : α) : MLList m α

Construct a singleton monadic lazy list from a single value.