Re-enable program execution with mutable memory

[maxgillett]

In order to address issue #7, memory references in the Rust runner were changed from mutable to immutable references (and memory writes changed to reads), so that trace execution could be parallelized. This was a breaking change for generating proofs directly from compiled Cairo programs, without a provided register and memory trace. The code should be refactored to allow both parallelization and memory writes (perhaps using interior mutability), so that both of the operation modes described in the readme can be supported.