feat: add more lemmas about lists

These are 'leftover' lemmas I created while trying to prove
`String.splitOn_of_valid`. See
leanprover-community#743.

Batteries/Data/List/Lemmas.lean

@@ -24,6 +24,10 @@ namespace List
 theorem head_cons_tail : ∀ (l : List α) (hne : l ≠ []), l.head hne :: l.tail = l
   | _::_, _ => rfl
 
+theorem singleton_head_eq_self (l : List α) (hne : l ≠ []) (htl : l.tail = []) :
+    [l.head hne] = l := by
+  conv => rhs; rw [← head_cons_tail l hne, htl]
+
 /-! ### next? -/
 
 @[simp] theorem next?_nil : @next? α [] = none := rfl
@@ -489,6 +493,9 @@ end Diff
 
 /-! ### prefix, suffix, infix -/
 
+theorem singleton_prefix_cons (a) : [a] <+: a :: l :=
+  (prefix_cons_inj a).mpr nil_prefix
+
 theorem ne_nil_of_not_prefix (h : ¬l₁ <+: l₂) : l₁ ≠ [] := by
   intro heq
   simp [heq, nil_prefix] at h