Add RFC for Luau function and table types

rfcs/function-and-table-types.md

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+# Add primitive function and table types
+
+## Summary
+
+Add types for "real" functions and tables.
+
+## Motivation
+
+Some APIs require "real" functions and tables, not just things that
+"look functiony" (e.g. tables with a `__call__` metamethod) or "look
+tabley" (e.g. instances of classes). This RFC adds types for those.
+
+## Design
+
+Add:
+
+* a type `table`, inhabited by Luau tables (but not class instances), and
+* a type `function`, inhabited by Luau functions (but not class methods or
+tables with metamethods).
+
+Luau functions with known source and targe types are now an intersection type `function & (T) -> U`.
+
+Luau tables with known properties are now an intersection type `table & { p : T }`.
+
+We may want to provide syntax sugar `function(T) -> U` and `table{ p : T }` for these, since they will probably be quite common.
+
+We should audit APIs to see which ones accept functiony or tably arguments, and which ones want real functions and tables.
+
+## Drawbacks
+
+Another bit of complexity budget spent.
+
+## Alternatives
+
+Stick with the current imprecision.