a st_blksize analog for filestat

[sunfishcode]

Some applications rely on struct stat to contain a st_blksize field to determine what size buffer to use. filestat doesn't currently provide this information.

It may also be useful to have a field providing the size of the file without any "holes" included, which would let libc implement a st_blocks field. POSIX's st_blocks is in 512-byte units (regardless of the st_blksize value), which is awkward, but if WASI itself simply provides the size in bytes, libc can divide by 512 to satisfy POSIX.