Safetensors changes

[swfsql]

• Makes the safetensors module private.
  • Doesn't get exported on the preamble, avoiding a naming clash with the safetensors external crate for apps that use dfdx.
• Change how and when the period . is inserted.
  • This should make it closer to how the fields are accessed in the code.
• For load, read, save, write safetensor(s), add a _with method:
  • Requires load/read to decide whether it should skip missing tensors;
  • Requires load/read/save/write to decide how should keys be mapped.
  • This makes it easier to load/save from/to models that were not saved by dfdx and thus contains a different safetensors key/location structuring.
• Allow models to get built from a safetensors byte array data (not from reading a file).

This doesn't have much testing nor documentation updates, so this PR should be considered a draft.