Sync to upstream/release/529

[rblanckaert]

• Adds a currently unused x86-64 assembler as a prerequisite for possible future JIT compilation
• Fix a bug in table iteration (closes Possible table iteration bug #504)
• Improved warning method when function is used as a type
• Fix a bug with unsandboxed iteration with pairs()
• Type of coroutine.status() is now a union of value types
• Bytecode output for tests/debugging now has labels
• Improvements to loop unrolling cost estimation
• Report errors when the key obviously doesn't exist in the table