[WIP] Validate jumpdests

[0xkarmacoma]

Validate the legality of jump destination targets

Previously, you could jump anywhere in bytecode (inluding non-jumpdests or the middle of a PUSH), we did not validate that the bytecode jumped on a valid jumpdest

See #251

TODO:

• [X] test_illegal_jump
• [X] implement jumpdest validation for JUMP
• [ ] test_illegal_jumpi
• [ ] implement jumpdest validation for JUMPI