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particularly useful for zipfiles and f16s

- FROM /path/to/{safetensors,pytorch}
- FROM /path/to/fp{16,32}.bin
- FROM model:fp{16,32}

This fixes issues with blob file names that contain ':' characters to be rejected by file systems that do not support them.

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