Fix double quotes on non-interpolated strings

Strings that don't use interpolation shouldn't be double-quoted. This slows down compilation because the compiler needs to look through the double-quoted string for any interpolation it might have. But more importantly it's quicker to read because single-quotes guarantees that there's no 'magic' going on in that string.

This MR doesn't fix everything - there's a ton of more complex strings that I couldn't handle with a simple regexp - but I think that 661 changed lines is enough for a single commit ;)