Fixes and tests

[lacraig2]

This pull request introduces several improvements and new tests to the dwarffi project, focusing on correctness, robustness, and coverage of edge cases. The changes include bug fixes in core library logic and the addition of comprehensive tests for backend interaction, bitfield invariants, zero-copy buffer handling, and type normalization.

Bug fixes and logic improvements:

• Improved symbol acceptability logic in _is_acceptable by requiring a non-zero address instead of allowing either a valid address or type info. This tightens the criteria for accepting symbols.
• Enhanced robustness of constant data decoding in get_decoded_constant_data by explicitly checking for None and enabling strict validation in base64.b64decode.

New and enhanced test coverage:

Backend and memory interaction:

• Added test_backend_read_write_sizes_and_addresses to verify that backend read and write operations are called with correct addresses and sizes, using a custom RecordingBackend.

Bitfield and struct invariants:

• Introduced test_bitfield_preserves_other_bits_across_patterns to ensure that setting bitfields does not affect unrelated bits in the containing struct, across a variety of initial patterns.

Zero-copy and buffer handling:

• Added tests in test_zero_copy.py to confirm that zero-copy views share underlying data correctly and that read-only buffers are properly rejected when writable access is required.

Type normalization and packing:

• Added tests to verify that type string normalization is consistent (e.g., whitespace and prefixes) and that manual byte packing matches the output of ffi.new.