Swap definitions of CHERI stack vs stack-frame size, which could otherwise
lead to hitting the guard page during sandbox failure handling.
Add a kernel debugger command, 'show cheristack' to print out the trusted
stack for the current thread.

Implement an alternative trap handler used when a sandbox triggers a trap
-- e.g., a page fault or capability bounds failure.  Forceably CReturn to
the caller, and set v0/v1 to indicate a failure has ocurred (not currently
part of our defined ISA/ABI).  We can now tolerate a variety of memory
errors / failure modes for sandboxes without terminating the containing
process.  This is not particularly elegant: we'd really like this
functionality to be in userspace, to allow language run-time specific
unwinding and failure handling, but it appears to work quite well in
practice, and should be sufficient for a demo.  It does not yet handle
sandbox timeouts.