Cranelift: Add heap_load and heap_store instructions (#5300)

* Cranelift: Define `heap_load` and `heap_store` instructions

* Cranelift: Implement interpreter support for `heap_load` and `heap_store`

* Cranelift: Add a suite runtests for `heap_{load,store}`

There are so many knobs we can twist for heaps and I wanted to exhaustively test
all of them, so I wrote a script to generate the tests. I've checked in the
script in case we want to make any changes in the future, but I don't think it
is worth adding this to CI to check that scripts are up to date or anything like
that.

* Review feedback

cranelift/codegen/src/ir/entities.rs

@@ -394,6 +394,34 @@ impl Heap {
     }
 }
 
+/// An opaque reference to some out-of-line immediates for `heap_{load,store}`
+/// instructions.
+///
+/// These immediates are too large to store in
+/// [`InstructionData`](super::instructions::InstructionData) and therefore must
+/// be tracked separately in
+/// [`DataFlowGraph::heap_imms`](super::dfg::DataFlowGraph). `HeapImm` provides
+/// a way to reference values stored there.
+///
+/// While the order is stable, it is arbitrary.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+#[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
+pub struct HeapImm(u32);
+entity_impl!(HeapImm, "heap_imm");
+
+impl HeapImm {
+    /// Create a new `HeapImm` reference from its number.
+    ///
+    /// This method is for use by the parser.
+    pub fn with_number(n: u32) -> Option<Self> {
+        if n < u32::MAX {
+            Some(Self(n))
+        } else {
+            None
+        }
+    }
+}
+
 /// An opaque reference to a [WebAssembly
 /// table](https://developer.mozilla.org/en-US/docs/WebAssembly/Understanding_the_text_format#WebAssembly_tables).
 ///