Move execution-plan types Value and Primitive into a new crate

This way, modeling-cmds can import those types and implement the traits. In the future, I will write a derive macro for deriving Value on modeling-cmd types.

Cargo.toml

@@ -2,6 +2,7 @@
 resolver = "2"
 members = [
     "execution-plan",
+    "execution-plan-traits",
     "modeling-cmds",
     "modeling-session",
     "unit-conversion-derive",

execution-plan-traits/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "kittycad-execution-plan-traits"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+serde = { version = "1.0.193", features = ["derive"] }
+thiserror = "1.0.51"
+uuid = "1.6.1"
+
+[lints]
+workspace = true

execution-plan-traits/src/lib.rs

@@ -0,0 +1,70 @@
+//! TODO
+// pub use impl_value_on_primitive_ish;
+
+pub use self::primitive::{NumericPrimitive, Primitive};
+
+#[macro_use]
+mod primitive;
+
+/// Types that can be written to or read from KCEP program memory.
+/// If they require multiple memory addresses, they will be laid out
+/// into multiple consecutive memory addresses.
+pub trait Value: Sized {
+    /// Store the value in memory.
+    fn into_parts(self) -> Vec<Primitive>;
+    /// Read the value from memory.
+    fn from_parts<I>(values: &mut I) -> Result<Self, MemoryError>
+    where
+        I: Iterator<Item = Option<Primitive>>;
+}
+
+/// TODO
+#[derive(Debug, thiserror::Error, Default)]
+pub enum MemoryError {
+    /// Something went wrong
+    #[error("Something went wrong")]
+    #[default]
+    MemoryWrongSize,
+    /// Type error, memory contained the wrong type.
+    #[error("Tried to read a '{expected}' from KCEP program memory, found an '{actual}' instead")]
+    MemoryWrongType {
+        /// What the KittyCAD executor expected memory to contain
+        expected: &'static str,
+        /// What was actually in memory
+        actual: String,
+    },
+    /// When trying to read an enum from memory, found a variant tag which is not valid for this enum.
+    #[error("Found an unexpected tag '{actual}' when trying to read an enum of type {expected_type} from memory")]
+    InvalidEnumVariant {
+        /// What type of enum was being read from memory.
+        expected_type: String,
+        /// The actual enum tag found in memory.
+        actual: String,
+    },
+}
+
+/// Macro to generate an `impl Value` for the given type `$subject`.
+/// The type `$subject` must be "primitive-ish",
+/// i.e. something that can be converted Into a Primitive and TryFrom a primitive
+#[macro_export]
+macro_rules! impl_value_on_primitive_ish {
+    ($trait:ident, $subject:ident) => {
+        impl $trait for $subject {
+            fn into_parts(self) -> Vec<Primitive> {
+                vec![self.into()]
+            }
+
+            fn from_parts<I>(values: &mut I) -> Result<Self, MemoryError>
+            where
+                I: Iterator<Item = Option<Primitive>>,
+            {
+                values
+                    .next()
+                    .ok_or(MemoryError::MemoryWrongSize)?
+                    .to_owned()
+                    .ok_or(MemoryError::MemoryWrongSize)?
+                    .try_into()
+            }
+        }
+    };
+}