run_block_generator()

[arvidn]

Add a higher level function to run block generators. This includes the generator ROM program. Raising the abstraction level allows the function to return the full cost of the generator (including the byte-cost). Previously, we could only return the CLVM and conditions cost.

This patch is a step towards porting the generator ROM to rust, which would have these advantages:

1. We could run blocks with a lower memory footprint by resetting the allocator in between each puzzle
2. We could record the CLVM cost per puzzle (which helps with identical spend deduplication, which needs to track cost per spend)
3. We could fail invalid blocks faster, bu parsing and partially validating the conditions after each puzzle. i.e. we wouldn't need to run all puzzles first, and then parse and validate their output.

[richardkiss]

I did make some comments, but I'll leave it up to you as to if you feel like implementing any of them.