Parse src.cpp rules at build time

[lucic71]

Fixes #55

Each src.cpp from the rules/ dir is now parsed at build time, instead of on every invocation of cpp2rust. This gives a 5-8x speedup in running the unit tests in the Github runner. Translating a simple unit test like assert.cpp went down form 6 seconds to 0.1 seconds.

The new cpp-rule-preprocessor binary from cpp2rust/ is responsible for parsing one src.cpp at a time and to generate a ir_src.json in the same dir as the src.cpp file. The format of the json file is: f<n> / t<n>: { "to_string": <Mapper::ToString> }.

All src.cpp files are parsed in parallel, CMakeLists takes care of that. The check-rules target is now responsible for parsing the Rust rules and the C++ rules. The IR for src.cpp is not indexed in git. The IR for the Rust rules remains unchanged.

I ported the clang AST matcher form translation_rule.cpp into the new file rule_src_parser.cpp. As a result, translation_rule.cpp is only responsible for loading IR JSONs.