coremem_types: add Peano numbers

not used yet, but will be by list/enum operations

crates/types/src/compound/mod.rs

@@ -1 +1,2 @@
 pub mod list;
+pub mod peano;

crates/types/src/compound/peano.rs

@@ -0,0 +1,15 @@
+pub struct PNext<P>(P);
+pub struct P0;
+pub type P1 = PNext<P0>;
+
+pub trait Peano { }
+
+pub trait PeanoNonZero {
+    type Prev: Peano;
+}
+
+impl Peano for P0 { }
+impl<P: Peano> Peano for PNext<P> {}
+impl<P: Peano> PeanoNonZero for PNext<P> {
+    type Prev = P;
+}