Bug 12418 – Add module for C-strings support

C-strings processing is a special and common case so: 1. C-strings should be supported with both performance and usability. 2. There should be a dedicated module for C-strings (instead of adding such functions here and there in other modules). Current state: there is no good support for C-strings in Phobos, there is slow and broken `toStringz` (Issue 12417), and no standard way to make many common operations, like converting returned C-string to string and releasing its memory or creating a C-string from string using an allocation function. So I propose to add `unstd.c.string` [1] module to Phobos which include all use-cases I have seen implementing (correct and fast in contrast to existing ones like GtkD (yes, it's both incorrect and slow because of tons of GC allocations)) C library wrappers. [1] http://denis-sh.bitbucket.org/unstandard/unstd.c.string.html

Seems to be add meanwhile in std.internal.cstring.