RFC: Deprecate table.getn/foreach/foreachi (#715)

[Rendered](https://github.com/Roblox/luau/blob/zeux-rfc-deptable/rfcs/deprecate-table-getn-foreach.md)

rfcs/deprecate-table-getn-foreach.md

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+# Deprecate table.getn/foreach/foreachi
+
+## Summary
+
+Mark table.getn/foreach/foreachi as deprecated
+
+## Motivation
+
+`table.getn` was deprecated in Lua 5.1 that Luau is based on.
+
+`table.getn(x)` is equivalent to `rawlen(x)` when `x` is a table; when `x` is not a table, `table.getn` produces an error. It's difficult to imagine code where `table.getn(x)` is better than either `#x` (idiomatic) or `rawlen(x)` (fully compatible replacement). However, `table.getn` is slower and provides yet another way to perform an operation, leading new users of the language to use it unknowingly.
+
+`table.foreach` and `table.foreachi` were deprecated in Lua 5.2.
+
+`table.foreach` is equivalent to a `for .. pairs` loop; `table.foreachi` is equivalent to a `for .. ipairs` loop; both may also be replaced by generalized iteration. Both functions are significantly slower than equivalent `for` loop replacements, are more restrictive because the function can't yield, and result in new users (particularly coming from JS background) unknowingly using these thus producing non-idiomatic non-performant code.
+
+In both cases, the functions bring no value over other library or language alternatives, and thus just serve as a distraction.
+
+## Design
+
+We will mark all three functions as deprecated. The only consequence of this change is that the linter will start emitting warnings when they are used.
+
+Removing support for these functions doesn't provide any measurable value and as such is not planned in the foreseeable future because it may cause backwards compatibility issues.
+
+## Drawbacks
+
+None
+
+## Alternatives
+
+If we consider table.getn/etc as supported, we'd want to start optimizing their usage which gets particularly tricky with foreach and requires more compiler machinery than this is probably worth.