Add clang-tidy wish-list

Author: danakj

tidy/README.md

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+# Things that we want a clang-tidy for
+
+## Closure concept usage
+* Templates restricted as Fn are received as const&-ref
+* Templates restricted as Fn are invoked as lvalue `f()`
+* Templates restricted as FnMut are received by value or as &-ref
+* Templates restricted as FnMut are invoked as lvalue `f()`
+* Templates restricted as FnOnce are received as &&-ref or by value
+* Templates restricted as FnOnce are invoked as rvalue `sus::move(f)()`
+
+## References to closures
+* FnOnceRef, FnMutRef and FnRef types only appear as lvalues as ParamVarDecl,
+  and are not returned.
+
+## Long-lived closures
+* Think about how to rewrite FnBox to allow lambdas with capture if we use a
+  clang-tidy to ensure arguments are not stored as references carelessly.
+
+## Mutable references
+* Passing an lvalue to a function receiving an lvalue-reference should require
+  the argument to be wrapped in `sus::mref()`. Probably exceptions for generic
+  code.