This PR adds a new `powdr_expression` crate, containing a simplified
version of `powdr_ast::analyzed::AlgebraicExpression`. The idea here is
that this decouples our Autoprecompiles code from the PowdrVM & PIL
codebase. This will allow us to gradually change the expression type
according to our needs. See #2832 and #2833 as work-in-progress
examples.
Differences to previous `AlgebraicExpression`:
- Generic over the reference type (but
`powdr_autoprecompiles::legacy_expression` adds `AlgebraicReference` and
defines `type AlgebraicExpression<T> = ActualAlgebraicExpression<T,
AlgebraicReference>`, for now)
- Removed `AlgebraicExpression::{PublicReference,Challenge}` variants
- Removed `AlgebraicBinaryOperator::Pow` variant
- `degree_with_cache()` -> `degree()`, as we don't have intermediate
polynomials anymore
I included a `powdr_ast::analyzed::AlgebraicExpression <-->
powdr_autoprecompiles::legacy_expression::AlgebraicExpression`
conversion, which is only used to interact with the `powdr_pilopt`
crate, which still is used by the APC codebase and the PowdrVM / PIL
codebase.
