New package: base-encode 0.3.1

ITP: #1142263

Needed to de-vendor node-rollup v4 (#1092721): rollup's bundled rust/xxhash
crate uses base_encode::to_string() unconditionally, and that crate is compiled
into the SWC WASM parser during the node-rollup build.

- lintian clean; only expected tags:
  - distribution-and-changes-mismatch (release.sh finalises the changelog)
  - I: extended-description-is-probably-too-short (we keep upstream's own short
    Cargo.toml description verbatim)
  - I: package-contains-documentation-outside-usr-share-doc (README.md in the
    registry dir; standard for debcargo crates)
- autopkgtest: PASS
- No non-DFSG content
- One patch: bump-rand-dev-dependency.patch — Debian ships rand 0.8/0.10 but not
  0.7; the rand dev-dependency (used only by examples/base36.rs via
  rand::random(), which is API-stable) is bumped 0.7 -> 0.8 so the
  autopkgtest dependency is satisfiable.
