Variable interpolation in strings

[Justin Whear]

Jonathan M. Davis' recent post about string mixins reminded me of 
something I put together a couple of months ago: variable interpolation 
in strings http://dpaste.dzfl.pl/5689d535

Some of the projects I work on have lots of embedded queries which are 
assembled on the fly (not just values but column and table names). Using 
concatenation tends to break the queries up excessively, while using 
format with lots of arguments makes them hard to modify. So I took a page 
from PHP's book and implemented "$foo" style expansion.

My original thought was to allow cool syntax like x!"embedded variable: 
$foo" but template instantiations don't have access to variables from 
instantiation scope (unless you pass them explicitly via alias, of 
course).  Thus the need for string mixins.
The `inContext` version does take a struct or object which constitutes 
the set of available variables.

My ugly parsing code could be replaced (maybe, haven't actually tested 
it) by the nice version(none)'d function which uses regexs if match() was 
available for CTFE.

Justin