Implement SaddSat and SsubSat for the interpreter

Implemented `SaddSat` and `SsubSat` to add and subtract signed vector
values, saturating at the type boundaries rather than overflowing.

Changed the parser to allow signed `i8` immediates in vectors as part of
this work; fixes #3276.

Copyright (c) 2021, Arm Limited.

cranelift/codegen/src/ir/immediates.rs

@@ -26,6 +26,12 @@ impl IntoBytes for u8 {
     }
 }
 
+impl IntoBytes for i8 {
+    fn into_bytes(self) -> Vec<u8> {
+        vec![self as u8]
+    }
+}
+
 impl IntoBytes for i16 {
     fn into_bytes(self) -> Vec<u8> {
         self.to_le_bytes().to_vec()