Reconsider `as_c_str` function for working with strings over FFI

[bstrie]

@brson and @pcwalton both brought up concerns regarding the current as_c_str API in this pull: #6729 . @graydon also had some comments regarding this in http://thread.gmane.org/gmane.comp.lang.rust.devel/4151 . Figure out what design is best and implement it.

[Kimundi]

EDIT: Moved the comment to #7235 (comment)

[emberian]

closing, since @erickt's work seems to have done this.