Add exception mechanism to 'switched' vm

[ncos]

• Add 'ret' statement (but still no 'call' statement)
• Add function call stack (exceptions are treated as calls)
• Add a couple of tests

[ncos]

• Add new instruction to set exception handler address
• Remove the necessity to start with jump

[grigory-rechistov]

Accepted, this closes #3. Kept in the mitrokhin-task03 branch.