This makes a couple of changes to the behavior/implementation of `halide_malloc()`:

* Currently, halide_malloc must return a pointer aligned to the maximum meaningful alignment for the platform for the purpose of vector loads and stores. This PR also adds the requirement that the memory returned must be legal to access in an integral multple of alignment >= the requested size (in other words: you should be able to do vector load/stores "off the end" without causing any faults).

* Currently, the `halide_malloc_alignment()` function is used to determine the default alignment; this cannot be overridden by user code (well, it can be, but the override will have no useful effect). It is intended to be "internal only" but is used in at least one place outside the runtime (apps/hannk). This change removes the call entirely, in favor of a call that is harder to access from outside the runtime and much less likely for end users to attempt to call. (It also changes apps/hannk to stop using it.)