Refactor internals for speed, fix bugs, bump to 1.8.7

[bobverity]

Internal refactor with no change to documented behaviour (verified against a golden-master snapshot of every exported function), removing duplicated code and large per-string dplyr overhead. Biggest speedups in drop_read_counts and position_from_variant_string (now pure string operations).

Bug fixes:

• get_component_variants() no longer errors on valid strings that combine a phased het locus with a homozygous locus (e.g. pfcrt:72_73:C|S_V).
• check_position_string() now validates every distinct input, not just the first n_unique original elements (invalid strings after duplicates were being skipped).
• check_variant_string()/check_position_string() now carry the failure reason in the error condition (conditionMessage) rather than an empty stop().

Behaviour changes:

• drop_read_counts() and position_from_variant_string() preserve the input format rather than re-normalising it (position ranges and concise amino-acid notation are kept; genes are still sorted alphabetically).

Other:

• Moved test-case CSVs from inst/extdata to tests/testthat/testdata and read them with readr::read_csv (strips the UTF-8 BOM that had silently broken the validator tests); added guards so fixture-load failures are loud.
• Removed the unused tidyr dependency.