equation_to_mpoly

It would be helpful to have a function that converts a equation specification `y = x^2` or `y == x^2` (in a string or quoted) into a character representation `"y - x^2"`, or quoted. This would also affect __bertini__ (e.g. `bertini::variety()`) and __algstat__ (e.g. `algstat::ggvariety()`).

Start by locating the existing equation-handling entry points in mpoly, then inspect how bertini::variety() and algstat::ggvariety() would use the conversion. Done means accepting string or quoted specifications with either = or == and producing the requested character representation such as "y - x^2".

Written by the indexing model from the issue text.