Add a way to (de)serialize Dex values

Trying out the dxbo as a way to get parameters into dex but it seems broken (or maybe just incomplete).

Any other ideas how to get parameters in? Load dxo works but is really slow.

```haskell
dat : (Fin 2 => Float & Float) = ([10.0, 20.0], 10.0)
dump dxbo "examples/bin-tmp.dxbo" dat
> Compiler bug!
> Please report this at github.com/google-research/dex-lang/issues
>
> Prelude.undefined
> CallStack (from HasCallStack):
> error, called at libraries/base/GHC/Err.hs:78:14 in base:GHC.Err
> undefined, called at src/lib/Serialize.hs:238:16 in dex-0.1.0.0-31vc1WNSYI0CcnFyEWUMxN:Serialize
```

Read src/lib/Serialize.hs around line 238 and compare the existing dxbo and dxo paths; reproduce the reported dump failure with the example value. Done means providing a supported way to serialize and deserialize Dex values, including the reported tuple/function value, while preserving the existing loading behavior.

Written by the indexing model from the issue text.