Added a table of control characters in ASCII table

There is a new enumerated type $(REF ControlChar, std, ascii). It has
values for every control character in ASCII table and has a base type
of `char`, allowing for alternative markup of strings with control
characters:
-------
import std.ascii, std.conv;
with (ControlChar) assert(text("Phobos", us, "Deimos", us, "Tango", rs) == "Phobos\x1FDeimos\x1FTango\x1E");
-------
