Warn on recursive bindings

When you accidentally write something like let x = .. x ... it can take hours to realize where you made your mistake. This hits me once in a while, and my colleagues often.

I'd propose e.g. -Wrecursive-bindings that says:

warning: [-Wrecursive-bindings]
    Recursive binding for `x` in

    let x = length x

This applies to let, where and top-level pattern bindings.

I believe that in practice, I only actually use real recursive bindings once in a while. So I might be bold enough to encourage enabling it in -Wall for a future major GHC release.

With the compromise that if you have the warning enabled but in one specific place, you want a recursive binding, you can use the ~ tilde to say "I really mean it", e.g.

let ~ones = 1 : ones

That seems like a nice balance to say "I know what I'm doing in this case". So the warning could be more helpful, like:

warning: [-Wrecursive-bindings]
    Recursive binding for `ones` in

    let ones = length ones

    If intentional, use the tilde marker on the name like this:

    let ~ones = length ones

In Intero if I were to implement a prototype of this check, I'd probably do this, after renaming:

1. Use SYB to collect all variable bindings from the pattern.
2. Use SYB to listify all mentions of any of these variables in the RHS and any guards or where clauses.

If the list is non-empty, then trigger the error. A transformation function [Name] -> Pat -> Pat would provide the AST with the offending name(s) tilded as ~x for use in the error message.

If there's general agreement, I could implement this change.

• *EDIT**: mutually recursive bindings apply here too. So let x = y; y = x by a regular occurs check.