Add support for 128-bit integers

Only compile in this support if rustc is new enough, otherwise continue
to omit the bindings for older-than-1.26 rustc

Closes #143
diff --git a/src/stable.rs b/src/stable.rs
index 6c06fc4..0d8aac9 100644
--- a/src/stable.rs
+++ b/src/stable.rs
@@ -678,6 +678,12 @@
         f64_suffixed => f64,
     }
 
+    #[cfg(u128)]
+    suffixed_numbers! {
+        u128_suffixed => u128,
+        i128_suffixed => i128,
+    }
+
     unsuffixed_numbers! {
         u8_unsuffixed => u8,
         u16_unsuffixed => u16,
@@ -691,6 +697,12 @@
         isize_unsuffixed => isize,
     }
 
+    #[cfg(u128)]
+    unsuffixed_numbers! {
+        u128_unsuffixed => u128,
+        i128_unsuffixed => i128,
+    }
+
     pub fn f32_unsuffixed(f: f32) -> Literal {
         let mut s = f.to_string();
         if !s.contains(".") {