Always keep serial numbers as verbatim

[YDX-2147483647]

Resolves #506

In the current docs/file-format.md, the type of serial-number is specified as string, not numeric or string:

hayagriva/docs/file-format.md

Line 331 in 0eaceab

| **Data type:** | string or dictionary of strings |

However at present, hayagriva tries to parse serial numbers as numeric numbers when .bib → .yaml and when .yaml → CSL variable.
Usually it fails, so serial numbers will be kept as verbatim as expected; but when it succeeded, the serial number is transformed unexpectedly.

#506 gives an example of the latter case: USDL-26-0599.
Without knowing it's a serial number, we may interpret it as a range from number 26 (with prefix USDL-) to number 599, and format it incorrectly as USDL-26–599 (USDL-26, en dash, 599).

To fix it, this PR removes the parsing logic for serial numbers from interop.rs (.bib → .yaml) and taxonomy.rs (.yaml → CSL variable).

Additional info

Leading zeros in serial numbers should be kept, but I'm not sure whether leading zeros in volumes should be kept.
Therefore, I intentionally construct test cases that do not involve volumes with leading zeros in this PR.
I or someone else may deal with it in future PRs.