Deprecate the use of Value::Hint in kimchi poseidon

[katat]

The Value::Hint is going to be deprecated (https://github.com/zksecurity/noname/issues/38). The only place it is still being used is the poseidon function in kimchi backend.

We would need to figure out how to replace the following hint closure function:

https://github.com/zksecurity/noname/blob/222e57572a5b5cd4adfe6ea9fb32a33dfc4616ed/src/backends/kimchi/builtin.rs#L100-L115

[mimoo]

let's implement this directly in the compute_val match statement and create a Value::Poseidon. Or we could also deprecate it for now (as poseidon could be implemented differently depending on the backend...

[mimoo]

related to https://github.com/zksecurity/noname/issues/38 btw