Sync to upstream/release/535

[zeux]

• Implement support for unknown / never types (RFC Add RFC for never and unknown types #434)
• Improve type inference for higher order generic functions
• Improve type checking for combinations of intersections and generics
• Fix a bug where type arena ownership invariant could be violated, causing stability issues
• Fix a bug where internal type error could be presented to the user
• string.gmatch now infers returned types based on the pattern when using a literal
• Fix a bug with negative zero in vector components when using vectors as table keys
• Slightly reduce interpreter I$ footprint