Closes#2535
See the referenced issue for longer discussion - here's the synopsis.
Consider this program
```
app "test" provides [ nums ] to "./platform"
alpha = { a: 1, b: 2 }
nums : List U8
nums =
[
alpha.a,
alpha.b,
]
```
Here's its IR:
```
procedure : `#UserApp.alpha` {I64, U8}
procedure = `#UserApp.alpha` ():
let `#UserApp.5` : Builtin(Int(I64)) = 1i64;
let `#UserApp.6` : Builtin(Int(U8)) = 2i64;
let `#UserApp.4` : Struct([Builtin(Int(I64)), Builtin(Int(U8))]) = Struct {`#UserApp.5`, `#UserApp.6`};
ret `#UserApp.4`;
procedure : `#UserApp.nums` List U8
procedure = `#UserApp.nums` ():
let `#UserApp.7` : Struct([Builtin(Int(I64)), Builtin(Int(U8))]) = CallByName `#UserApp.alpha`;
let `#UserApp.1` : Builtin(Int(U8)) = StructAtIndex 1 `#UserApp.7`;
let `#UserApp.3` : Struct([Builtin(Int(I64)), Builtin(Int(U8))]) = CallByName `#UserApp.alpha`;
let `#UserApp.2` : Builtin(Int(U8)) = StructAtIndex 1 `#UserApp.3`;
let `#UserApp.0` : Builtin(List(Builtin(Int(U8)))) = Array [`#UserApp.1`, `#UserApp.2`];
ret `#UserApp.0`;
```
What's happening is that we need to specialize `alpha` twice - once for the
type of a narrowed to a U8, another time for the type of b narrowed to a U8.
We do the specialization for alpha.b first - record fields are sorted by
layout, so we generate a record of type {i64, u8}. But then we go to
specialize alpha.a, but this has the same layout - {i64, u8} - so we reuse
the existing one! So (at least for records), we need to include record field
order associated with the sorted layout fields, so that we don't reuse
monomorphizations like this incorrectly!
When I created this (at the very beginning of the Wasm backend),
I didn't really have a clear reason for it. I just thought it might end up
making sense treat heap pointers differently from numbers, somehow.
But the semantic differences between pointers and other numbers is not relevant
to WasmLayout. The semantics are clear from where the Symbol appears in the IR.
Also we were storing heap pointers in locals, for no real reason.
And the fact that it's *different* meant a lot of new cases in match expressions,
to do the exact same thing as Primitives but with a pointless difference.
Until now, we haven't really used this variant in any of our tests.
But the refcount pointer needed it... and everything broke!
