Some new features

I was thinking of some final improvements I wanted to make before abandoning this for some other wild hare, and this is what I came up with:

• Made cell ranges first-class collections within Backwards. This should be a little more memory efficient.
• Allow numbers to be written with 'E' (scientific) notation.
• Close the number system with the projective closure. Now, I can treat the numbers as if they were floating point.
• Added the EVAL function.

One of the ideas floating in my head for the "Son of DeciCalc" is to allow the user to select the number system, and this moves the code in that direction.

Included libraries (from Cygwin for mingw64-x86_64):

• GMP : 6.3.0-1
• libstdc++ : 11.4.0-1
• ncurses : 6.4-20231121