Consider adding instructions to read memory of the parent context

One of the consequences of #948, is that we would get access to the parent context ID via `prnt_ctx` column. This means, we would be able to execute operations which read values from (and even write values to) parent memory.

While writing values into parent context may lead to potential security issues, reading values should be OK in most cases, and it could simplify passing large amounts of data from the caller to the callee.

The instructions could be something like `pmem_load` and `pmem_loadw` (to mimic `mem_load` and `mem_loadw`).