Improve Symbolic Object Hashing to Include Metadata and Symtype

[bowenszhu]

This pull request enhances the hashing of symbolic objects to incorporate metadata and symtype. This addresses potential hash collisions between mathematically equivalent expressions with differing metadata or symtypes, which helps #658.

To maintain compatibility with the existing ordering of symbolic objects, which relies on the original hash function, the original implementation has been preserved as hash2.

The data structures for Add and Mul utilize Dict. However, because Dict internally uses Base.hash for key placement, this change introduces some ordering differences, leading to failures in several existing tests. Further investigation and potential mitigation strategies for these test failures are needed as mentioned in #667. This PR opens the discussion for how to best address these ordering issues while benefiting from the improved hash incorporating metadata and symtype.

[jpfairbanks]

Will this also change equality to include metadata and symtype? If you have two variables with the same name, but different metadata, those should probably be nonequal.

Edit: This is actually #669