eval and string interpolation.

String interpolation happens in double quotes (a la Perl/Ruby) but not single quotes. An expression can be interpolated between curly braces. Also, a bunch more things are now expressions. (Blocks -- if, while -- are not expressions though.)

'eval' will evaluate the text contained in an expression. Hooray? Each eval thing has its own scope, so variables declared inside the eval can't be referenced outside of it. The eval can reference variables outside of itself, though.
Merge branch 'master' of https://github.com/broomweed/avocado