Formal review of std.buffer.scopebuffer

[Dicebot]

Initial announcement : 
http://forum.dlang.org/post/ld2586$17f6$1@digitalmars.com
Documentation        : 
http://www.walterbright.com/tmp/html/scopebuffer.html
Code                 : 
https://github.com/D-Programming-Language/phobos/pull/1911

Starting formal review thread for Walter's proposal.

Linked pull request already has some ongoing discussion and is 
worth investigating. In this thread please focus on those 
high-level properties of reviewed module:

  - Is it useful for standard library?
  - Does it fit into Phobos design?
  - Is documentation / API clear enough for you?
  - Will it be possible to keep it within its current API without 
breaking changes?
  - Are there any major oversights in existing implementation?

Smaller style / implementation comments are also welcome but less 
important at current stage.

Review will last for 2 weeks (ending on March 30) but can be 
prolonged upon explicit request.