✨ Make blocks and fields hashable

[MiWeiss]

Summary

Block.__eq__ and Field.__eq__ were defined without __hash__, making all blocks and fields unhashable (set(library.entries) raised TypeError). This PR decides and implements the hash/equality semantics:

• Equality stays value-based (unchanged): same type + same __dict__.
• Hashes use a stable subset of the eq-attributes — (type, start_line, raw) for blocks, (type, start_line, key) for fields — so equal objects always hash equal (hash contract holds). Mutable/unhashable attributes (fields, parser_metadata, field values that may be lists/NameParts) are excluded, so a block doesn't lose its hash when middleware mutates values. As with any hashable-mutable design, mutating the hashed attributes while an object sits in a set/dict is unsupported.
• Library.remove/replace are now identity-first: with two equal blocks in a library (e.g. identical comments), the exact instance passed is removed/replaced; equality matching remains as fallback, so existing code keeps working.

Tests

• Equal entries/strings/fields hash equal; set deduplication and dict lookup work (including fields with unhashable list values).
• remove/replace with two equal comments operate on the passed instance, verified via is.

Fixes #537

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