Implement VariantRecord#calc_number

[multimeric]

See #669

• Expose bcf_get_value_count as VariantRecord#calc_number
• Add tests for this
• Some extra docstrings

[multimeric]

Some questions about my implementation:

• My editor auto-formatted the code, should I undo that?
• I make users specify INFO or FMT as a string to determine the type of field. Should I expose an enum/constant for these values instead? If strings, should I use FORMAT instead of FMT? Do we have a precedent?

[bioinformed]

Re editor reformatting: Yes-- you'll need to undo all of that for this MR to be considered.
Re API for FMT vs. INFO: I left some suggestions in #669.