[kernel][cmd] Don't cast between function pointer types

Such casts are not valid in standard C++.

Change-Id: I99e5c95ee3c802bd9051904fffc3ac98aeec93d2

[kernel] Check alloc result in kernel benchmarks

Reduce kernel benchmark buffer size from 8MB to 3MB.

Kernel heap will reject allocations larger than 4MB (less
header). Prior to this change, kernel benchmarks were attempting to
allocate 8MB. This would result in a crash because they failed to
check the alloc result before using it.

Tested this change by running them and seeing them complete.

ZX-1955 #done

Change-Id: I56876d3cd97b5e9ee6f332bbd90081a1a814a9a9

[kernel][tests] Fix printf() format string warnings

Enable -Wformat warnings for this directory by removing -Wno-format.

This involves:
* Size fixes for correctness/strictness.
* Signedness fixes, because we compile the kernel with
-Wformat-signedness (which GCC supports but Clang does not support).

Change-Id: Id1f3a73f4dbeee319e2aa80b05cc6648280dafd5

[kernel] Remove kernel/app module

This is no longer used at all.
The kernel/app/tests code doesn't use the "app" infrastructure.
So move it to kernel/tests.

Change-Id: I6cd8ec1bf569fd95940e5ebf4dc5f640b265eb69