Get rid of experimental code for handling "deriving"
corrected documentation of keepIf and friends
change Mouse.isClicked to adhere to its documentation
Use Build.Print to show errors
cosmetic: get rid of a (>>) in favor of a do-block
Rename the dummy paths file so that it does not register as C code!
Add check that all ports appear in the "main" module

It is unclear what it means if ports are used in sub-modules, so to be
cautious, we want to rule it out in the first release. As people use
the API it’ll become clear how sub-module ports are necessary and what
problems they might cause.
Attempt to address #463

@ jvoigtlaender, does this clarify things enough?
Try to address #464
Merge pull request #462 from jvoigtlaender/Mouse.isClicked-correction

change Mouse.isClicked to adhere to its documentation
Revert "Merge pull request #462 from jvoigtlaender/Mouse.isClicked-correction"

This reverts commit 26b7fc617f68301ad936929e3db410438b13954e, reversing
changes made to 9be0abd01889256a337e829d9c938f9df28633ae.
Merge pull request #461 from jvoigtlaender/keepIf-docu

corrected documentation of keepIf and friends
Change semantics of keepWhen/dropWhen.

They no longer propagate the right Signal when the left Signal
changes.
Uncomplicate keepWhen implementation
Remove old JS dropWhen/keepWhen