Remove taskchampion source from this repo (#3427)

* move taskchampion-lib to src/tc/lib, remove the rest
* update references to taskchampion
* Use a top-level Cargo.toml so everything is consistent
* apply comments from ryneeverett

xtask/src/main.rs

@@ -0,0 +1,35 @@
+//! This executable defines the `cargo xtask` subcommands.
+//!
+//! At the moment it is very simple, but if this grows more subcommands then
+//! it will be sensible to use `clap` or another similar library.
+
+use std::env;
+use std::fs::File;
+use std::io::Write;
+use std::path::{Path, PathBuf};
+
+pub fn main() -> anyhow::Result<()> {
+    let manifest_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR")?);
+    let workspace_dir = manifest_dir.parent().unwrap();
+    let arguments: Vec<String> = env::args().collect();
+
+    if arguments.len() < 2 {
+        anyhow::bail!("xtask: Valid arguments are: `codegen`");
+    }
+
+    match arguments[1].as_str() {
+        "codegen" => codegen(workspace_dir),
+        _ => anyhow::bail!("xtask: unknown xtask"),
+    }
+}
+
+/// `cargo xtask codegen`
+///
+/// This uses ffizz-header to generate `lib/taskchampion.h`.
+fn codegen(workspace_dir: &Path) -> anyhow::Result<()> {
+    let lib_crate_dir = workspace_dir.join("src/tc/lib");
+    let mut file = File::create(lib_crate_dir.join("taskchampion.h")).unwrap();
+    write!(&mut file, "{}", ::taskchampion_lib::generate_header()).unwrap();
+
+    Ok(())
+}