const-oid: encoding round trip failure

[tarcieri]

Caught by proptest:

https://github.com/RustCrypto/formats/actions/runs/14329602501/job/40162382341

proptest: If this test was run on a CI system, you may wish to add the following line to your copy of the file.
cc 61bdeaa6cfc6707a0c4f3e9c6165d99d28042e78acb29b0ca7f747c169e83e74

thread 'round_trip' panicked at const-oid/tests/proptests.rs:31:1:
Test failed: assertion failed: `(left == right)` 
  left: `"1.1.1.270000000"`,
 right: `"1.1.1.1564544"` at const-oid/tests/proptests.rs:40.
minimal failing input: s = "1.1.1.270000000"
	successes: 224

[tarcieri]

Now that const_mut_refs is stable and const-oid is MSRV 1.85, it's probably time to rewrite the encoder using mut refs, which should provide many opportunities for simplification