SafeUnorderedMap fixes

[fcecin]

• Implement operator=()
• Implement size()
• Fix erase-insert-commit bug in commit()
• Improve & fix operator=() testcase that was calling the copy constructor instead
• Add erase-insert-commit regression test
• Fix all testcase asserts that depended on the placeholder size() code

[jcarraror]

size() could be void since it's modifying a member variable. Hence, empty() function could compare member size_ directly. Everything else seems good 💯