[board, core, position] A simple static evaluation method for scoring positions

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.

position/src/lib.rs

@@ -1,11 +1,12 @@
 // Eryn Wells <eryn@erynwells.me>
 
+mod evaluation;
 mod position;
 
 #[macro_use]
 mod macros;
 
-pub use chessfriend_board::{fen, PlacePieceError, PlacePieceStrategy};
+pub use chessfriend_board::{PlacePieceError, PlacePieceStrategy, fen};
 pub use chessfriend_moves::{GeneratedMove, ValidateMove};
 pub use position::Position;