Add a way to convert ints to strings in interpolation

[johnynek]

Something like
"the count is $#{cnt}"

Where to draw the line is a question here though. If we add this, it should also work in pattern matching position, and that complicates string parsing and patterns.

[johnynek]

If we imagine these are format strings that are also parsers it seems somewhat compelling.

Maybe "$c{x}$i{y}$f{z}"

To print or parse characters, integers and floats.

I like the idea, but have to be a bit careful about string totality checking. I guess you can just always assume it is refutable.