Make hash helpers thread-safe (fixes #72)

[zejji]

Problem

ByteArrayExtensions and StreamExtensions compute hashes through single, process-wide static HashAlgorithm instances:

private static readonly Crypto.MD5 _md5 = Crypto.MD5.Create();
private static readonly Crypto.SHA256 _sha256 = Crypto.SHA256.Create();

A HashAlgorithm is not thread-safe, so calling these helpers from multiple threads throws:

System.Security.Cryptography.CryptographicException:
Concurrent operations from multiple threads on this type are not supported.

This is easy to hit in practice because InMemoryBlobStorage.Write hashes every write via data.MD5(), so writing blobs from multiple threads (parallel tests, concurrent request handling, etc.) fails intermittently. Reported in #72.

Fix

Hash with a per-call instance instead of a shared static:

• On .NET 5+, use the allocation-free, thread-safe one-shot MD5.HashData / SHA256.HashData.
• On netstandard2.0 / 2.1, create and dispose a per-call instance with using (consistent with the existing HMACSHA256 helper in the same file).

Only the instance lifecycle changes; the hash output is identical. The change covers all three affected helpers: ByteArrayExtensions.MD5, ByteArrayExtensions.SHA256, and StreamExtensions.MD5.

Tests

Adds:

• Known-value tests for ByteArrayExtensions.MD5/SHA256 and StreamExtensions.MD5, asserting the canonical vectors for "The quick brown fox jumps over the lazy dog".
• Concurrency regression tests for the byte[] helpers (Hash_helpers_can_be_called_concurrently, covering MD5 and SHA256) and the Stream helper (MD5_can_be_called_concurrently). Two threads hash concurrently, each looping enough that they reliably overlap inside ComputeHash (the race only trips while both threads are inside it at the same moment). These fail on the unfixed code (reproduced the CryptographicException in 30/30 runs) and pass with this fix, in a few tens of ms.

Verified the library compiles across every target framework (netstandard2.0;netstandard2.1;net7.0;net8.0;net9.0), so both branches of the #if build.

Fixes #72.