This adds two new utilities allocating arbitrary types as grants
to kernel::grant::Allocator, and splits the internal `alloc_unowned`
method into `alloc_raw` and `alloc_default_unowned` to allow allocating
non-`Default` values.

The new `alloc` allows allocating values not default-initializable,
such as references to AppSlices. I believe this is a reasonable change,
as no existing methods use `alloc`, and the new interface more closely
matches other Rust allocation interfaces, such as `std::boxed::Box`.

The new `alloc_n_with` then allows allocating a runtime-determined
number of values. This will be useful if, for example, the size of
allocation needed is determined by a variable given by a TockOS app.

This is motivated by a use case in the Rubble BLE capsule, storing
attributes. The number of attributes is usually known at compile-time
for the app, but not for the kernel. If the capsule ends up using an
array-like structure, `alloc_n_with` will be useful to allocate
app-specific memory meant for dealing with attributes the app
provides. If the capsule ends up using a linked-list-like structure,
then the new `alloc` will allow allocating list nodes which contain
non-Default data, such as `AppSlice`s. We don't necessarily need both
interfaces, but I believe that together they form a sane base for
everything one might need for dynamic memory allocation.

Signed-off-by: David Ross <David.Ross@wdc.com>

This also renames the function, as I think the closure makes it more of
a "_with" function than not.

Signed-off-by: David Ross <David.Ross@wdc.com>