TxGraph::balance's API is too complicated

```rust
pub fn try_balance(
&self,
chain: &C,
chain_tip: BlockId,
outpoints: impl IntoIterator,
mut trust_predicate: impl FnMut(&OI, &Script) -> bool,
) -> Result {
```

`OI` is unnecessary. The caller passes it to us and then we query it. It can just be:

```rust
pub fn try_balance(
&self,
chain: &C,
chain_tip: BlockId,
outpoints: impl IntoIterator,
mut trust_predicate: impl FnMut(&OutPoint, &Script) -> bool,
) -> Result {
```

If the caller wants to know the value of `OI` in trust predicate it can just look it up itself via the `OutPoint`.