Add functions for idxstats

Add functions to access inside hts_idx_t and retrieve stats.

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>
Improve hts_idx_get_stat to work for more file types

Add META_BIN macro to get meta bin rather than putting idx->n_bins + 1 everywhere.
Alter hts_idx_get_stat to use META_BIN rather than constant and work for BAI, CSI and tabix.

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>
Return 0/-1 rather than bool; parenthesise C macro

Returning -1 for failure is the existing htslib convention.
Merge API functions for index statistics (PR #83)