Fix block stack table

[plafer]

Fixes the block stack table with the following logic:

• on CALL: fn_hash contains the parent function hash; it is only set to callee_hash in the next row
• on SYSCALL: fn_hash also contains the parent function hash; it remains unchanged by a syscall
• on END for call/syscall: block stack logic resets the fn_hash columns to the parent fn hash only in the next row

[plafer]

Related: #483 for docs

[bobbinth]

Looks good! Thank you for fixing this!