Support Linux

[dfed]

This change required excluding @objc declarations from the build on Linux, no longer using NS* function calls, and not importing SQLite3 on Linux.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.01%. Comparing base (aa8536d) to head (20a058c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
- Coverage   99.02%   99.01%   -0.02%     
==========================================
  Files          11        8       -3     
  Lines         515      507       -8     
==========================================
- Hits          510      502       -8     
  Misses          5        5              

Files with missing lines	Coverage Δ
Sources/CacheAdvance/BigEndianHostSwappable.swift	100.00% <100.00%> (ø)