King's movement - castling

[lrozenblyum]

The castling is rather a complex topic, therefore I introduce a separate sub-issue of #65

(0) This is a move of the king and either rook of the same colour along the player’s first rank, counting as a single move of the king and executed as follows: the king is transferred from its original square two squares towards the rook on its original square, then that rook is transferred to the square the king has just crossed.

(1) The right to castle has been lost:
if the king has already moved, or
with a rook that has already moved.

(2) Castling is prevented temporarily:
if the square on which the king stands, or the square which it must cross, or the square which it is to occupy, is attacked by one or more of the opponent's pieces, or
if there is any piece between the king and the rook with which castling is to be effected.

[lrozenblyum]

It's easier to start from move execution itself than from conditions when it's allowed, so I'll start from position generation ...

[lrozenblyum]

Currently position generation cases:
a) white short
b) white long
c) black short
d) black long

[lrozenblyum]

Implementing #83 thus added castling both king-side and queen-side to list of allowed moves (via source-> destination square like "e1", "g1" not using 0-0 notation)

[lrozenblyum]

#91 reference added