[kernel][heap] Don't use heap for thread stacks

Kernel stacks are now allocated by vm_allocate_kstack.

Thread creation no longer allows you to bring your own
stack. thread_create and thread_create_etc now always create a stack
of size DEFAULT_STACK_SIZE.

Remove THREAD_STACK_BOUNDS_CHECK feature as it has been subsumed by
guard pages provided by vm_allocate_kstack.

Test: verified no leaks using "k vmm kaspaces" and "k thread_tests"

ZX-2509 #done

Change-Id: I8dbde5c8a6852775b9796e0c7f963289291822b2

[zircon][syscalls] Make zx_time_t and zx_duration_t signed types

This change is part of a hard transition and breaks the layers above.

zx_time_t and zx_duration_t are now int64_t. ZX_TIME_INFINITE is now
INT64_MAX.

A negative time value passed to a syscall is treated as any other time
value that's "in the past".

Test: CQ; new unit tests; built and ran topaz system on Eve

ZX-2100 #comment

Change-Id: I2a020cb2cf3dd7212608e24c1ee90cec58dd51ef

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

Such casts are not valid in standard C++.

Change-Id: I99e5c95ee3c802bd9051904fffc3ac98aeec93d2

[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