Replace MRB_ARGS_ANY usages with explicit argument specs

[Mukundhan-P]

Fixes #15

This PR replaces the remaining MRB_ARGS_ANY usages with explicit argument specifications.

Changes:

• filename_to_uri_string -> MRB_ARGS_REQ(1) | MRB_ARGS_KEY(1, 0)
• uri_string_to_filename -> MRB_ARGS_REQ(1) | MRB_ARGS_KEY(1, 0)
• create_reference -> MRB_ARGS_REQ(1) | MRB_ARGS_KEY(1, 0)

These signatures match the corresponding mrb_get_args definitions and make the API more explicit.

[gemmaro]

Thank you! Sounds good to me.