Additions: "Also" for side-effects and "debug" for printing debug messages

[nicorichard]

Creates a debug function which can be used to log print statements to aid in debugging

e.g.

Gen
    .from(1...5)
    .debug()
    .map { $0 * 2 }
    .debug("*2")
    .take(5)

will produce:

14:22:42.205: [GenythingTests/GenDebugTests.swift:8] -> 5
14:22:42.205: [*2] -> 10
14:22:42.205: [GenythingTests/GenDebugTests.swift:8] -> 4
14:22:42.205: [*2] -> 8
14:22:42.205: [GenythingTests/GenDebugTests.swift:8] -> 3
14:22:42.205: [*2] -> 6

In addition I've added also to the toolkit which was used to make debug possible.

also can be used to introduce side-effects during value generation, something that I wouldn't really recommend unless perhaps to implement a different logging/debugging solution.