No way to specify relative fuzz by parsing a String.

[rchillyard]

We would like to parse an expression such as:

• lazymath"9.81~1%"
• lazymath"9.81±1%"
• Eager("9.81~1%")
• Eager("9.81±1%")

Instead, we must write an expression such as Real(9.81, Some(RelativeFuzz(0.01, Box))).