previous layer creation was not ideal because:
1. it required reading the input file multiple times, once to calculate
the sha256 checksum, another to write it to disk, and potentially one
more to decode the underlying gguf
2. used io.ReadSeeker which is prone to user error. if the file isn't
reset correctly or in the right place, it could end up reading an
empty file
there are also some brittleness when reading existing layers else
writing the inherited layers will error reading an already closed file
this commit aims to fix these issues by restructuring layer creation.
1. it will now write the layer to a temporary file as well as the hash
function and move it to the final location on Commit
2. layers are read once once when copied to the destination. exception
is raw model files which still requires a second read to decode the
model metadata
