It turns out I was wrong and we can set the page size to the actual
page size used by the target architecture when dividing the binary
into chunks and calculating a hash of each chunk for embedding within
the adhoc code signature. This shaves of a considerable amount of bytes
since we divide the code signature section by at least 2x.

I've also unified the `write` interface of `CodeSignature` struct to
follow that used in every other bit of `MachO`; namely, the functions
now accept a writer instead of a buffer, therefore, there is no need
to manually track where to write each struct field anymore.