[M1] add manifest::write

CHANGELOG.md

@@ -25,3 +25,7 @@ All notable changes to cargoxx will be documented in this file.
   `[package]` / `[build]` and unknown top-level keys are rejected; reserved
   fields (`description`, `repository`, `[dev-dependencies]`, `[features]`,
   `[workspace]`) are accepted. `tests/manifest_parse.cpp` covers 17 cases.
+- `manifest::write(m, path)` serializes a `Manifest` as TOML using toml++.
+  Dependencies are emitted alphabetically (matches Cargo). Round-trip
+  property is exercised by `tests/manifest_write.cpp` (9 cases).
+  Defaulted `operator==` on the manifest structs supports comparison.