Fix bugs in BTREE-DELETE and SPLIT-BTREE-NODE.

Rename BTREE-DELETE to BTREE-DELETE-KEY and implement BTREE-DELETE for
btrees with non-unique keys.

Add stress test for btrees.

Implement the :MIN, :MAX, :INCLUDE-MIN, :INCLUDE-MAX and :ORDER arguments
for BTREE-MAP.

Add some more CL mirror functions like P-MAPCAR, P-MAPC, P-DELETE-IF, etcetera.