[Chronik] Add log! and log_chronik! macros

Summary:

1. Add CHRONIK log category to BCLog
2. Split log_println into log_print (logs unconditionally) and log_print_chronik (logs to category BCLog::CHRONIK)
3. Add chronik-util crate
4. Add log! (calls log_print) and log_chronik! macro (calls log_print_chronik)

Note: There is no (built-in) equivalent to the __FUNC__ macro in Rust, so we just put <chronik unknown> there. Capturing the current function name is possible but requires a bit of a hack (see https://stackoverflow.com/a/40234666).

Note: __FILE__ in C++ is relative to cwd (e.g. build), but file!() in Rust is relative to /. We haven't spent any time harmonizing this, as this is sufficient for debugging purposes.

Test Plan:

1. ninja
2. ./src/bitcoind -help: Under -debug option, it lists chronik
3. ./src/bitcoind -regtest: Still logs (only) "Starting Chronik..." as before
4. ./src/bitcoind -regtest -debug=chronik: Now also logs "Note: Chronik is not implemented yet. These logs are just for testing."
5. ./src/bitcoind -regtest -debug=chronik -logsourcelocations: Logs:

[chronik/chronik-lib/src/bridge.rs:11] [<chronik unknown>] Starting Chronik...
[chronik/chronik-lib/src/bridge.rs:12] [<chronik unknown>] Note: Chronik is not implemented yet. These logs are just for testing.

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12026