Change Composer into a struct

[moCello]

Summary

Currently we have the Composer trait that encodes the functionalities for composing a circuit.
Since we only ever implement this trait onto the Builder, we an morph the trait and struct into a single Composer struct. This comes with the added benefit that we can hide some of the trait functionalities from the public API that are now exposed but marked as deprecated (e.g. uninitialized, append_witness_internal, append_custom_gate_internal).