Consider collapsing `when`/`if` into a single keyword

[byorgey]

The difference between pattern matching and guards is already confusing, but maybe having two separate keywords to memorize makes it better, not worse. Even if both start with the same keyword there is still the presence or absence of the keyword is to disambiguate. One might worry the parsing is tricky but I don't think it will be that bad, just throw in some backtracking.

This was a suggestion from monochrom in the #haskell IRC channel.