Implement a new Evaluator struct that evaluates a Board and returns a score. This
evaluation mechanism uses only a material balance function. It doesn't account
for anything else.
Supporting this, add a Counts struct to the internal piece set structure of a
Board. This struct is responsible for keeping counts of how many pieces of each
shape are on the board for each color. Export a count_piece() method on Board
that returns a count of the number of pieces of a particular color and shape.
Implement a newtype wrapper around i32 called Score that represents the score of
a position in centipawns, i.e. hundredths of a pawn. Add piece values to the
Shape enum.
