allele[0] is the REF (allele[] pointers to the als block); all null terminated
REF+ALT block (\0-seperated)
FILTER keys in the dictionary
FORMAT and individual sample
ID
if set, indiv.s must be recreated on BCF output (TODO: make enum)
INFO
allocated size (high-water mark); do not change
Number of FILTER fields
variant number(???)
if set, shared.s must be recreated on BCF output (TODO: make enum)
$var and $var_type set only when set_variant_types called
variant type (TODO: make enum)
Variable-length data from a VCF record