This commit relocates the explorer code from the research directory to `bin/explorer` to include it as part of future releases.
### Summary of Updates:
#### Darkfi Project
- Updated `Cargo.toml` to include `bin/explorer/explorerd` as a workspace member
- Updated `Cargo.lock` to include the `explorerd` package
- Updated the `Makefile` to include `explorerd` in the build process
#### Explorer Daemon
- Renamed the project directory from `blockchain-explorer` to `explorer`
- Moved the explorer daemon source code to `bin/explorer/explorerd`
- Updated the cargo package name to `explorerd`
- Updated log statement targets from `blockchain-explorer::` to `explorerd::`
- Renamed the explorer configuration file to `explorerd_config.toml`
- Removed Halo2 patches as they are now included in the root package
- Changed default db_path to use explorerd instead of blockchain-explorer in the path
- Changed binary crate Arg structopt name from blockchain-explorer to explorerd
#### Explorer Site
- Moved the explorer site source code to `bin/explorer/site`
- Updated README.md to include new build instructions for explorerd
