Add expression evaluator to C programs

[williamalexakis]

Adds a mathematical expression evaluator to the C folder.

The program uses a basic recursive-descent parser I wrote to calculate math expressions with <math.h>.
It supports:

• + - * /
• Unary + -
• sin cos tan sqrt log exp
• Variables

Example usage:

./expression_eval "(x + y) * 2^3" x=4 y=15
152

[williamalexakis]

Hey, could this PR please be reviewed? Thanks.