Add better unicode support (Fraction slash/division slash)

[feefladder]

Since Rust supports full Unicode and because of the name of this crate, I think the fraction slash '⁄' is a better delimiter than the solidus '/' for display purposes. In any case, it should not be an error in parsing to use:

• Solidus: '/'
• Fraction slash: '⁄'
• Division slash: '∕'
• Division sign: '÷'

Here is a discussion I started on Rust internals. and here is a SO unicode discussion on fractions.