* refactor: keep torrent hash in std::array<char,20>
This replaces the hashString QString allocation with a compile-time
array that's included in sizeof the Torrent struct that owns it.
Explicitly delete the implied function to ensure we don't get bitten by
this again in the future.
Mark the string_views constexpr where possible so that we can avoid some
strlen() calls at runtime.
* refactor: make variant_headers reusable to qt app.
Torrent.cc's `change()` template methods are generically useful to deal
with tr_variant wrangling, but previously were only used in Torrent.cc.
This PR moves them into a new API `VariantHelpers.h` for use by Prefs,
Session, TorrentModel, etc.
