feat: Support formatting hex/oct/bin integers

[pastelmind]

Add methods that create Formatted<i64> in hexadecimal, octal, and binary representations. In accordance with TOML 1.0 spec, these forms can be created only if the integer is nonnegative. For hexadecimal, it is possible to create both uppercase or lowercase representation.

This PR does not change the AST and should be fully backwards-compatible.

Note: See https://github.com/toml-rs/toml/discussions/781#discussioncomment-11979453 for the discussion leading up to this PR. This approach is much more conservative than my initial attempts to create a new Integer type that manages the current representation (hex/oct/bin). However, said approach was brittle: it wasn't clear how to classify mixed-case hexadecimals (0xabCDef), or hexadecimals with no alphabets (0x1234). I eventually realized that this was a formatting problem and that adding custom constructors for Formatted<i64> would suffice.

[epage]

There is a lot of design discussion to be had on this. My general stance is PRs is for implementation discussion and all design discussion should be done in issues.

Since #812 has been created, I'd recommend pushing the discussion there.