The previous implementation suffered from four problems:
    1. try_lock in the basic irq_lock always succeeded.
    2. lock in the basic irq_lock always succeeded.
    3. try_lock in the wrapper irq_lock did not use the base try_lock.
    4. a foreign CPU could wrongly unlock a remote irq_lock.

These issues are mitigated in this patch.

Previously, we allocated a 512-byte buffer for every call of the
formatted panic function. This was heavily increasing the stack size for
a number of functions.

We now don't print to a statically allocated buffer, but rather use the
iterator-based formatting facility and a proxy iterator with a small,
self-flushing buffer. This reduces that static overhead of the formatted
panic function to by almost 500 bytes, while increasing possible message
fidelity, since we are not longer arbitrarily limited by the buffer.

The coding guidelines explicitly prohibit the use of "naked"/C-style
pointers. However, there were some prominent examples in the kapi and
the core kernel source. This changeset replaces them with the
appropriate smart pointer types.