make beneath infixl 6

[byorgey]

While playing with diagrams-ghci, mgsloan and I noticed that beneath did not have the same precedence as <> (or atop). This commit gives beneath the same fixity and precedence as atop, namely, infixl 6.

The only question is why atop is infixl, as opposed to <> which is infixr. I don't remember if I had a good reason for this when I initially gave atop its fixity.