Broken down the learnyouahaskell_test namespace to per-chapter namespaces.
Changed the Just Monoid implementation, nil is id now.
return and unit now depend on the *pure-context* local binding, which is
nil by default.
bind and mdo handle return and unit automatically using thread-local bindings. Added new namespace utils for keeping context-related and utility functions.
Changed the vararg implementations of bind and fapply functions in CurriedFn.