Implement hash consing for Sym

[bowenszhu]

• Hash consing #614

This PR implements hash consing for Sym as the first step towards applying hash consing across SymbolicUtils.jl.

Motivation:

Hash consing enables efficient memory usage and potential performance improvements by ensuring that only one unique copy of a symbolic expression exists in memory.

Implementation Details:

• A WeakValueDict is used to store existing symbolic objects. This choice ensures that symbolic objects without strong references are eligible for garbage collection.
• The BasicSymbolic struct is modified to be mutable to comply with the requirements of WeakValueDict (due to how Julia implements finalizers).
• A new "hash" function, hash2, is introduced to incorporate symtype into the hash calculation. This approach was chosen over modifying the existing hash function to avoid breaking many tests because ordering is dependent on hash values.
• A new isequal2 function is added for metadata comparison in addition to Base.isequal, because directly modifying Base.isequal breaks numerous tests in ModelingToolkit.jl.

Future PRs will apply hash consing to other BasicSymbolic subtypes.

[ChrisRackauckas]

Looks reasonable enough and the implementation is clean. Are there performance benchmarks to see if it's going on the right direction?

[AayushSabharwal]

Considering this matches hashes to symbolic variables, wouldn't it also need to handle hash collisions? If two Syms end up with the same hash, it would return a different one than what the user expects.

Also, if hashconsing is using a custom hash function could it also hash the metadata and not require that it be empty?

[YingboMa]

We need a dictionary that takes both hash and equal to handle collision.

[bowenszhu]

requesting review @AayushSabharwal @ChrisRackauckas