Phobos for Review: std.buffer.scopebuffer

[Walter Bright]

https://github.com/D-Programming-Language/phobos/pull/1911

This adds a package std.buffer, and the first entry in that package, 
std.buffer.scopebuffer.

ScopeBuffer is an OutputRange that sits on the stack, and overflows to 
malloc/free. It's designed to help in eliminating gc allocation by lower level 
functions that return buffers, such as std.path.buildPath(). With some judicious 
user tuning of the initial stack size, it can virtually eliminate storage 
allocation.

Using it is @system, but the user of ScopeBuffer can be @trusted.

An output range like this is a precursor to eliminating the excessive gc use by 
functions such as buildPath().