Changes to Rust options parsing to facilitate Pants NG

[benjyw]

• Allow passing a buildroot through to NativeOptionParser
• Replace uses of HashMap with BTreeMap, to allow the relevant structs to implement Hash
• Implement Eq and Hash on various structs, which in some cases has to be done manually. E.g., Config values are not naturally Eq because they can contain floats.
• Support secure (sha256) hashing of config file contents.
• Make some private symbols pub or pub(crate) so they can be used by ng code.
• Change the config file name to pantsng_*.toml when running in "ng" mode.