loop and branch control flow abstraction for (by-value) pipes

[nerditation]

I've added three pipe methods (names to be bikeshedded):

• Pipe::pipe_when(): abstraction of if-else

  this is the same as pipe_if() in add pipe_if method(s) #7

• Pipe::pipe_for_some(): abstraction of if-let-else for Option

  this is basically the pipe_map() proposed in this comment of add pipe_if method(s) #7

• Pipe::pipe_for_each(): abstraction of for-loop

these are mostly to deal with builder patterns where the builder type is implemented with a self receiver. I add a motivating example to the documentation of each method.

my questions:

• I don't have a use case for Result personally, so I didn't include a pipe_for_ok(). is this use case common? should I add it "just for completeness"?

• I find builder patterns with a &mut self receiver is generally less annoying with control flows, since you don't need mut bindings and re-assignment, nor the else clause, so I'm satisfied with tap_mut() for similar use cases, and decided to not add similar constructs for Tap. I'd like to hear other's opinion.