Add treap implementation

Add treap implementation

Provide macros to generate a type-specific treap implementation and
various functions to operate on it. It uses obj_pool.h to store memory
nodes in a treap.  Previously committed nodes are never removed from
the pool; after any *_commit operation, it is assumed (correctly, in
the case of svn-fast-export) that someone else must care about them.

Treaps provide a memory-efficient binary search tree structure.
Insertion/deletion/search are about as about as fast in the average
case as red-black trees and the chances of worst-case behavior are
vanishingly small, thanks to (pseudo-)randomness.  The bad worst-case
behavior is a small price to pay, given that treaps are much simpler
to implement.

>From http://www.canonware.com/download/trp/trp_hash/trp.h

[db: Altered to reference nodes by offset from a common base pointer]
[db: Bob Jenkins' hashing implementation dropped for Knuth's]
[db: Methods unnecessary for search and insert dropped]
[rr: Squelched compiler warnings]
[db: Added support for immutable treap nodes]
[jn: Reintroduced treap_nsearch(); with tests]

Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>