[FEATURE] Pseudo-Legal Movegen

[dannyhammer]

Description:
Modify the engine to use pseudo-legal movegen, calling a game.is_legal(mv) method to determine if a move is legal for the current position.

Depending on whether this is done before or after #70, this might affect chessie.

Relevant Link(s):

• Stormphrax's isLegal

Requirements:

• Implement function(s) for determining if a move is legal on a position
• Provide unit tests for is_legal function
• Implement function(s) for generating pseudo-legal moves
• Refactor search code to use pseudo-legal movegen