ast: Remove UOP/BOP and use ops as AST node type directly
Add -appendbi option
cli: Treat all paths relative to @file, unless they're absolute
Improve -appendbi output formatting
tests: Move "high-level" tests out of ast/ subdir

Not all of them are about just testing the AST, and it seems right to have
more feature-specific tests at the toplevel next to c/cpp parsing tests...
Add -removematch option