chess-perft.el: Add another test case with depth 5.

Now that our execution time has been reduced significantly, add a bigger
test case to improve the testsuite coverage.
"make perft" now takes 10m33s.
chess-pos-search*: New function.

Search for several pieces in one swipe of the board, returning an
alist of pieces and their indices.  Use this in chess-legal-plies
for the common case where we want all legal plies for a certain
color makes the testsuite time go down to 9m57s.