Add debug.id RFC

rfcs/function-debug-id.md

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+# `debug.id`
+
+## Summary
+
+Add `debug.id` which accepts a value and always returns the identifier unique to that value.
+
+## Motivation
+
+When debugging, it is often useful to compare identity. Usually `print` will print the memory address of the table or userdata, but when `__tostring` is overriden, the address becomes unobtainable.
+
+`rawequal` can compare equality between two arguments, but is not suitable for a human comparing output between two `print` calls or between keys or values in a table.
+
+## Design
+
+`debug.id(value: any)` always returns the identifier unique to that value, and is not possible to override.
+
+While tables and userdata have individualized metatables, also allowing functions and threads to be passed to `debug.id` would let their type-level `__tostring` be redefined. Strings are useful to accept because of long, similar strings.
+
+## Drawbacks
+
+None known.
+
+## Alternatives
+
+Lua has `string.format("%p", {})`, but this is an invalid format option in Luau and less obvious.
+
+If not implemented, it remains annoying to compare identity when dealing with tables or userdata that override `__tostring`.