Reduce string allocations in `ExecutionScopes`

[gabrielbosio]

Line 59 of exec_scopes.rs allocates String for every variable: https://github.com/lambdaclass/cairo-vm/blob/8d4858689e01d439633375d70b8dc12053680a6a/vm/src/types/exec_scope.rs#L59

Use Cow<'static, str> instead.