Use vectors in candle's compute expressions

This issue is about turning
https://github.com/CakeML/cakeml/blob/e12b14fc45b8a54f755331338bf4960e3bcf39a0/candle/prover/compute/compute_execScript.sml#L43-L51
```
Datatype:
ce = Const num
| Var num
| Monop (cv -> cv) ce
| Binop (cv -> cv -> cv) ce ce
| App num (ce list)
| If ce ce ce
| Let ce ce
End
```
into
```
Datatype:
ce = Const num
| Var num
| Monop (cv -> cv) ce
| Binop (cv -> cv -> cv) ce ce
| App num (ce vector)
| If ce ce ce
| Let ce ce
End
```

Start at candle/prover/compute/compute_execScript.sml, lines 43–51, and inspect how the ce datatype and App expressions are used elsewhere in the compute code. Trace the affected construction and consumption sites before editing. Done means App consistently stores its arguments as a vector and the relevant compute development remains valid.

Written by the indexing model from the issue text.