Improve comment formatting and other small miscellaneous changes

[pdil]

• Removed CustomDebugStringConvertible conformance on Bitboard as this was causing every UInt64 to debug print as a chess board (funny but not ideal).
  • Bitboard.chessString() can be used to obtain the same string.
• Improved documentation comment formatting.
• Removed unnecessary "safe" dictionary subscript for Bitboard dictionaries (replaced with default:)
• Removed redundant return keyword where possible.