Euterpea exports Control.Arrow by default

Because of how often arrows and their combinators are used in Euterpea,
made Euterpea export the Control.Arrow module by default.  Examples were
updated accordingly.
Simplified (and fixed a bug in) some Playground code.
Removed Euterpea.Examples.BasicSignalExamples altogether because it is
incomplete/unenlightening/does not compile.