Proposal: takeFront and takeBack

[Jonathan M Davis]

Okay, given the fact that takeFront wouldn't work with ranges like 
std.stdio.ByLine, the code as proposed won't work. So, here's an adjusted 
proposal:

https://github.com/jmdavis/phobos/commit/14b88d9d5528f8736ae6014013bba82367e83620

As suggested, I renamed takeFront and takeBack to consumeFront and consumeBack 
respectively, but now they're in std.array and only apply to arrays.

hasConsume has been added to std.range and tests whether a particular range 
defines consumeFront (and if it's a bidirectional range, whether it defines 
consumeBack).

So, rather than just using consumeFront for any and all range types as would 
be nice (but can't be done due to a few screwy ranges like ByLine - but such 
is life), a range-based function which wishes to use consumeFront or 
consumeBack will test with hasConsume whether the range that they're operating 
on defines it (similar to what we do with hasSlicing for slicing). So, it 
unfortunately will require special casing on the part of any function looking 
to use consumeFront or consumeBack, but it _does_ allow us to operate on the 
ranges which support it more efficiently if we decide to go to the effort of 
special casing them.

If no one has any major objections to this scheme or can provide a better 
proposal, I'll create a pull request from what I have.

- Jonathan M Davis