When a test fails due to a difference in output between recording and replay, print the diff of the results.
Add assertions to check that the encoded event data field is wide enough for its data.
Fix raw-dump header to match reality.
Make tracee writes to rr's stderr/stdout non-interruptible.

This avoids situations where multiple threads concurrently write to
stdout/stderr, in which case rr does not know which order to echo them in
during replay.

Resolves #1201.
Microoptimize syscallbuf initialization to avoid redundant gettid() syscall.
Remove spurious return statement.