Mass refactor. Clang-format. Start adding tests. Use FetchContent.

.clang-format

@@ -0,0 +1,21 @@
+---
+Language: Cpp
+BasedOnStyle: Google
+
+AccessModifierOffset: -4
+AlignAfterOpenBracket: AlwaysBreak
+AllowShortBlocksOnASingleLine: false
+AllowShortFunctionsOnASingleLine: Inline
+BinPackArguments: false
+BinPackParameters: false
+BreakBeforeBraces: Allman
+BreakConstructorInitializers: AfterColon
+BreakInheritanceList: AfterColon
+ColumnLimit: 100
+ConstructorInitializerIndentWidth: 4
+ContinuationIndentWidth: 4
+DerivePointerAlignment: false
+IndentWidth: 4
+PointerAlignment: Middle
+ReflowComments: false
+...