basic version of for-comprehensions

[linusnorton]

This PR adds a basic implementation of Scala's for-comprehension.

It will only apply flatMap and map. Any filtering must be done manually.

Example usage can be seen in the tests but I would be happy to add documentation.

There is one limitation in that the type information is of the final result is lost and returned as Option<any>. In theory it should be Option<B> where B is the result type of the last function. Unfortunately it is not possible to get the type information of the last element of an array defined by a rest argument.

[linusnorton]

@shogogg thanks for merging, let me know if you want me to update the docs

[shogogg]

@linusnorton Thanks to PR, and please give me the PR that update docs if you can.