Enumeratee *map methods returns a complex object

This is a proposal to change Enumeratee API to avoid complex compilation errors.

When type error occurs with enumeratee.map, the compilation errors a bit complex to read and understand. This is a proposal to change their signature and simplify error messages and scaladoc.

As you will see in second commit, api changes ... and some user code must change accordingly. While changes are trivial, it should be noted.