Support block defaults for cattr and mattr

[dicnunz]

Summary

• Adds block defaults to cattr/cattr_reader/cattr_writer/cattr_accessor
• Mirrors the same block-default behavior for mattr helpers
• Adds focused module tests for reader/accessor defaults and write-after-default behavior

IssueHunt bounty:

• mattr/cattr (aka attribute accessors) to keep up with the times #258
• https://issuehunt.io/repos/341331/issues/258

Testing

• bundle install --path vendor/bundle --without guard
• bundle exec ruby-test -Ilib/core test/core/module/test_mattr.rb
• bundle exec ruby-test -Ilib/core test/core/module
• ruby -c lib/core/facets/module/mattr.rb
• direct Ruby smoke for cattr_accessor and mattr_accessor block defaults
• git diff --check

Note: a plain bundle install --path vendor/bundle tries to install the optional guard group and fails on this machine because ffi-1.17.4-x86_64-darwin requires Ruby >= 3.0, while the system Ruby here is 2.6.10. Installing without the guard group was enough for the test suite commands above.