Remove or rework the iree_hal_buffer_fill/read/write/etc functions

[benvanik]

The iree_hal_buffer_* functions for directly manipulating the buffer are not great. Vulkan doesn't have these (so we have to map memory), Metal doesn't have them or even a workaround that doesn't involve creating a command buffer and submitting it, and they are not guaranteed to work (as some backends may not support mapping memory).

Instead, we could make these functions take a command buffer to append operations to, or make them do the create command buffer + record + submit and wait themselves. Removing them entirely is probably best as they'll never be a good idea (can't work on device-local memory, inefficient on host-visible memory, etc).