Handle bcf_*_vector_end  - DO NOT MERGE YET

1. Modified SharedFieldIterator and IndividualFieldValueIterator to handle bcf_*_vector_end values. This is done by passing the end value of
the pointer in the constructor and modifying the "==" operator of the
iterator. The idea is that 2 iterators x,y are equal iff
 (a) x and y are identical  OR
 (b) (x == end && *y == bcf_*_vector_end) || (y == end && *x == bcf_*_vector_end)
2. Inserted test case fields for variable length vectors
htslib does use bcf_str_vector_end, just not by name. Its value is just 0, and htslib 0-pads string format fields when the strings are of different lengths.

bcf_str_missing, on the other hand, does indeed seem to be unused. htslib encodes a literal '.' to mean missing.