Cleanup and improvements (#194)

- Lots of changes for migrating C code to C++
- Improved `Lockable` in several ways like adding `withLock()` (+ tests)
- Improved `Semaphore` a bit for improved readability, and also added some tests
- Upgrade Linux machine in GitHub Actions so that we can compile with a newer GCC
- Simplification of WiFi connection
- Updated funding options
- (and more)

.github/FUNDING.yml

@@ -10,6 +10,6 @@ liberapay: # Replace with a single Liberapay username
 issuehunt: # Replace with a single IssueHunt username
 lfx_crowdfunding: # Replace with a single LFX Crowdfunding project-name e.g., cloud-foundry
 polar: # Replace with a single Polar username
-buy_me_a_coffee: # Replace with a single Buy Me a Coffee username
+buy_me_a_coffee: bytewelder
 thanks_dev: # Replace with a single thanks.dev username
 custom: # Replace with up to 4 custom sponsorship URLs e.g., ['link1', 'link2']