Allow to specify seed in block handler

[fehiepsi]

Currently, block hander does not expose message to outer seed handler. Users have to seed the blocked program explicitly. Hence the blocked program is no longer stochastic. To make it stochastic, users have to

• call prng_key primitive to get a seed
• seed the blocked program by that key

This pattern happens repeatedly in the code base and recently appears in the autoguide list PR #1644 .

I think we can make such procedure automatic for users.