Bug fixes

• d502c4a: Improve binary expression handling to not redundantly copy symbols that are constant since they cannot be modified.
• 93b1ed0: Add UnityEngine.Object to to whitelist filter in exposure tree
• 93b1ed0: Make the colors a little more readable on light theme for the exposure tree
• fc2f279: Add explicit errors for using is and as in U# since Udon does not yet expose what's needed to implement them properly, reported by @Xiexe
• a1d612e: Fix error when using script export on Udon Sharp Programs to Udon Assembly Programs on some types of heap symbol types, reported by 753
• 7f01386: Improve error handling during compile to avoid getting stuck with the UdonBehaviour infinitely trying and failing to compile a behaviour in some cases, reported by Mimi