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/opts.rs

@@ -9,8 +9,8 @@ pub use crate::ir::immediates::{Ieee32, Ieee64, Imm64, Offset32, Uimm32, Uimm64,
 pub use crate::ir::types::*;
 pub use crate::ir::{
     dynamic_to_fixed, AtomicRmwOp, Block, Constant, DynamicStackSlot, FuncRef, GlobalValue, Heap,
-    Immediate, InstructionImms, JumpTable, MemFlags, Opcode, StackSlot, Table, TrapCode, Type,
-    Value,
+    HeapImm, Immediate, InstructionImms, JumpTable, MemFlags, Opcode, StackSlot, Table, TrapCode,
+    Type, Value,
 };
 use crate::isle_common_prelude_methods;
 use crate::machinst::isle::*;