Revert "Remove const on all side_arg"

This reverts commit 511ee756cf8aa9259489b4386feb2911bb58c20a.

Re-introduce constness of side_arg to ensure that side_calls are pure.

This will facilitate integration with various runtimes and eliminate
corner-cases.

We plan on copying side argument vectors within libside when translating
from visitor types to flattened types.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>