v0.3.1: Breaking Changes!

Commit notes:

Rename Equation to Calculator and make root of file to improve mental model of what the struct actually is.
Split Infix and PostfixEquation into separate files.
Make Calculator initializer take in function pointers to registrar functions instead.
Add errorDescription function.
Rename PostfixEquation.fromInfixEquation to init
Make Operator and methods public