Async payment channel

See test dir for more usage examples. It demonstrates the work of all functions.
you can compile using stdlib_test.fc to replace my_address() and get_balance() calls with mocks but unfortunately, it does not work for now() because if compiler bug (function name collides with its opcode), you'll need to disable those calls in code manually to avoid errors.

We store:
init (1 bit) flag to initialize the contract
timeout (32 bits) timeout in second for exit
timestamp (48 bits) UNIX time in seconds
pubkA (256 bits) pub key of user A
pubkB (256 bits) pub key of user B
fA (1 bit) flag registry for A
fB (1 bit) flag registry for B
X_a (214 bits) nanograms registry for A
X_b (214 bits) nanograms registry for B

The total size is 1023 and it is enough to store in 1 Cell. 214 bits is enough to rotate all balance of the TON network for 5e45 times, so, it is safe.

In function challenge accept_message happens before signature check. Anybody can deplete channel by sending 'challenge' messages (actually looks like it will be made automatically by replaying: there is no replay protection as well).