[all] Remove dependencies on non-freestanding libc

This is the first of two rather big changes to clean up includes
throughout the project. In this commit, the implicit semi-dependency on
libc that bonnibel adds to every module is removed. Previously, I was
sloppy with includes of libc headers and include directory order. Now,
the freestanding headers from libc are split out into libc_free, and an
implicit real dependency is added onto this module, unless `no_libc` is
set to `True`. The full libc needs to be explicitly specified as a
dependency to be used.

Several things needed to change in order to do this:

- Many places use `memset` or `memcpy` that cannot depend on libc. The
  kernel has basic implementations of them itself for this reason. Now
  those functions are moved into the lower-level `j6/memutils.h`, and
  libc merely references them. Other modules are now free to reference
  those functions from libj6 instead.
- The kernel's `assert.h` was renamed kassert.h (matching its `kassert`
  function) so that the new `util/assert.h` can use `__has_include` to
  detect it and make sure the `assert` macro is usable in libutil code.
- Several implementation header files under `__libj6/` also moved under
  the new libc_free.
- A new `include_phase` property has been added to modules for Bonnibel,
  which can be "normal" (default) or "late" which uses `-idirafter`
  instead of `-I` for includes.
- Since `<utility>` and `<new>` are not freestanding, implementations of
  `remove_reference`, `forward`, `move`, and `swap` were added to the
  `util` namespace to replace those from `std`, and `util/new.h` was
  added to declare `operator new` and `operator delete`.

src/kernel/memory.cpp

@@ -2,27 +2,4 @@
 
 namespace std {
     enum class __attribute__ ((__type_visibility("default"))) align_val_t : size_t { };
-}
-
-// Implementation of memset and memcpy because we're not
-// linking libc into the kernel
-extern "C" {
-
-void *
-memset(void *s, uint8_t v, size_t n)
-{
-    uint8_t *p = reinterpret_cast<uint8_t *>(s);
-    for (size_t i = 0; i < n; ++i) p[i] = v;
-    return s;
-}
-
-void *
-memcpy(void *dest, const void *src, size_t n)
-{
-    const uint8_t *s = reinterpret_cast<const uint8_t *>(src);
-    uint8_t *d = reinterpret_cast<uint8_t *>(dest);
-    for (size_t i = 0; i < n; ++i) d[i] = s[i];
-    return d;
-}
-
-}
+}