[storage] Introduce storage_units and migrate Fxfs

Introduce the storage_units crate (//src/storage/lib/units) to represent
power-of-two block sizes and system page sizes as zero-cost typed
abstractions (GenericBlockSize, ValueBlockSize, and PAGE_SIZE).

Block sizes are guaranteed to be powers of two, replacing expensive
64-bit integer division and modulo operations with single-cycle bit
shifts and masks. Standard arithmetic, comparison, and formatting traits
are implemented, along with overflow-safe alignment helpers (align_up,
align_down, align_up_to_blocks, and is_aligned). On Fuchsia, PAGE_SIZE
caches the system page mask using an atomic to avoid repeated
zx::system_get_page_size() calls.

Migrate Fxfs to use BlockSize and PAGE_SIZE across the filesystem.

TAG: agy
Change-Id: I99fcec912f96284a4834277e9c0a3db17996bdf5
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1807218
Reviewed-by: Chris Suter <csuter@google.com>
Commit-Queue: Chris Drouillard <cdrllrd@google.com>