Make manifest serialization deterministic

Make manifest serialization deterministic

Fixes #4326

`cargo package` (and so `cargo publish`) parses a crate’s `Cargo.toml`,
makes some modifications, and re-serializes it.
Because the `TomlManifest` struct uses `HashMap`
with its default `RandomState` hasher,
the maps’ iteration order changed on every run.

As a result, when using `cargo vendor`,
updating a dependency would generate a diff larger than necessary,
with non-significant order-changes obscuring significant changes.

This replaces some uses of `HashMap` with `BTreeMap`,
whose iteration order is deterministic (based on `Ord`).