Implement ClassicAllocator for compiler S-expressions

[prozacchiwawa]

Summary

• add an SExpAllocator adapter using Rc<SExp> as its node pointer type
• preserve source locations during native atom and pair operations
• restrict the embedded CLVM allocator to import/export boundaries
• generalize atom buffers to support compiler-owned and borrowed atom data

Testing

• cargo test
• cargo check --all-targets
• focused source-location import/export round-trip test
• cargo clippy --all-targets -- -D warnings (blocked by 225 pre-existing test lint errors; no diagnostics in changed files)