Parse insertlane and extractlane instruction formats.

These instruction formats take immediate lane index operands. We store these as
u8 fields and require them to be in decimal format in the source. No hexadecimal
lane indexes are supported.

tests/parser/tiny.cton.ref

@@ -14,3 +14,10 @@ function select() {
 ebb0(vx0: i32, vx1: i32, vx2: b1):
     v0 = select vx2, vx0, vx1
 }
+
+function lanes() {
+ebb0:
+    v0 = iconst.i32x4 2
+    v1 = extractlane v0, 3
+    v2 = insertlane v0, 1, v1
+}