Add `WriteXdr::to_xdr_len()` for zero-allocation size computation

[tomerweller]

What

Add an to_xdr_len(&self) -> usize method to the WriteXdr trait that computes the serialized XDR byte length without allocating memory or performing I/O.

Why

Several downstream consumers serialize XDR values only to measure their byte length, then discard the bytes. For example, soroban-env-host's rent computation in e2e_invoke.rs allocates a Vec<u8> per ledger entry just to get buf.len(). For a typical Soroban transaction with ~10 footprint entries this results in ~40 unnecessary allocations per transaction in the ledger close hot path.

A dedicated to_xdr_len() method would allow callers to get the serialized size using only arithmetic over in-memory values — no allocation.

[leighmcculloch]

to_xdr_len, because the data has to be converted into the length, rather than it is static or preknown information that is cheap to extract.

[tomerweller]

@leighmcculloch, I changed the name to to_xdr_len