Add panic hook

[evgenykuzyakov]

Added STD panic hook. Now we expose Rust errors from Wasm through panic_utf8:

Examples:

unreachable macro:

Scheduling a call: panic-test.test_panic({})
Error:  TypedError: Transaction Byy8Hg5te6i9LtgCMcg8uVHzHnJmfTKBZJT5Aj7wPnEL failed. Smart contract panicked: panicked at 'internal error: entered unreachable code: AHAHA', src/lib.rs:29:9

invalid arguments:

Scheduling a call: panic-test.set_allowance({})
Error:  TypedError: Transaction 3GE7tCd2bg1GYhgXXUE7Y5wArLXKYujCQ8qvn6mVwjnC failed. Smart contract panicked: panicked at 'called `Result::unwrap()` on an `Err` value: Error("invalid type: null, expected a string", line: 0, column: 0)', src/libcore/result.rs:1084:5

Fixes #72

[evgenykuzyakov]

I also need to add a hook for initializer, 1 sec

[lexfrl]

great!