Input evaluation

You can now evaluate inputs, e.g. (3+2)*6 outputs 30. This works by
recursively evaluating the AST nodes — operator precedence comes for free
from the tree structure.

All variables found in the input are collected, and the user is prompted
to assign a value to each one. This somewhat defeats the purpose of
variables for now, but it’s a stepping stone — variables become essential
once symbolic differentiation or a proper UI come into play.