Improve LedgerSnapshot entry read/write performance

[mootz12]

What

Replaces the Vec-backed ledger entries storage with a new LedgerEntries struct combining a HashMap (O(1) keyed access) and a Vec<Box> (insertion-order preservation for serialization). Also adds count_entries(), cleans up the serde_with dependency for this type, and adds new tests.

This is a breaking change for consumers of LedgerSnapshot, as ledger_entries is now a private struct, and data can only be accessed through helper functions implemented on the LedgerSnapshot struct.

Why

Entry lookups previously did a linear O(n) scan through the Vec on every read/write — with (extremely) large snapshots this compounds significantly across all contract operations during tests. The HashMap brings those to O(1).

Closes #1781

Known limitations

• Improvements weren't as impactful as I'd hope. In testing across different repos, I saw almost no change for most contracts in integration style tests that record test results in snapshots. In general, the read/write performance of the snapshot does not appear to be the bottleneck here.
• Individual read/write performance was significantly improved for large snapshots (>10MB)

• Explore https://crates.io/crates/indexmap

[Copilot]

Pull request overview

This PR improves LedgerSnapshot performance during tests by replacing linear scans over a Vec of entries with a new LedgerEntries storage that provides O(1) keyed lookup while still preserving insertion order for JSON serialization.

Changes:

• Replaced LedgerSnapshot’s Vec-backed entry storage with LedgerEntries (HashMap + insertion-ordered key Vec) and added count_entries().
• Updated snapshot writing logic in Env to use count_entries() instead of iterating to count.
• Added/expanded tests covering ledger info and entry set/update/remove + serialization behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
soroban-sdk/src/env.rs	Uses the new count_entries() API for fast “empty snapshot” checks.
soroban-ledger-snapshot/src/lib.rs	Introduces LedgerEntries storage and count_entries(), refactors serde handling.
soroban-ledger-snapshot/src/tests.rs	Adds tests for ledger info and entry update/remove/roundtrip behavior.