Add block support to eval_ir_main

eval_ir_main only supports functions at the moment. https://github.com/google/xls/issues/534 discusses adding procs. We also need block support.

We have the ([block interpreter](https://github.com/google/xls/blob/main/xls/interpreter/block_interpreter.h)) which provides an API for interpreting blocks. It needs to be tied into eval_ir_main.

The couple big complications with blocks is multi-output and notion of cycles. We'll need some way of specifying particular inputs at particular cycles and expecting/extracting outputs similarly.