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ARROW-10322: [C++][Dataset] Minimize Expression

This PR replaces the `Expression` class hierarchy with a simpler discriminated union of:
- literal values
- field references
- call expressions which simply wrap a function name, a vector of arguments, and options.

```c++
Expression add_1_to_i32 = call("add", {field_ref("i32"), literal(1)});
```

This reduces the overhead of supporting new compute functions in dataset filters: execution and validation against a schema are already implemented and tested in `compute::`. Only serialization and equality comparison need to be manually wired up, and only if the function requires nontrivial function options.

TODO replace projection with expressions as well. The `struct` function is provided to replace `RecordBatchProjector`, invocable with `call` as with any other compute function:
```c++'
Expression projection = call("struct", {field_ref("f32"), add_1_to_i32},
                             StructOptions{"f32_renamed", "i32 + 1"});
```

Closes #8894 from bkietz/10322-Minimize-Expression-to-a-

Lead-authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Co-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>