More demos and QOL

Try it out: https://scratch.mit.edu/projects/1387804691/

Demos

• Math lexer, parser and interpreter: New demo that lets you input math expressions (eg: 1 + 1) and get the result.
• 3d cube renderer: a spinning 3d cube and player controls

QOL

• Smarter inlining cost calculation
• A math centered optimization for removing identities
• Get rid of PromoteToGlobals (temp)
• GC documentation
• Codegen fixes (receiver functions generate the same block, causing scratch to get confused)
• Stack size optimization (reuse stack slots of the same type)
• Fix crash when the garbage collector is disabled