XSB Src (SVN)-old

in curr_sym fixed a bug in predicate_property with properly handling tables.
in tables added max_ans as a tabling option.

Minor changes in tests (and output).
Added tests for interning (hash-consing) big_arity terms.

New implementation of index trees. Smaller, slightly faster, simpler.

Fixed bugs in handling of big-arity terms (arity > 255).

Added predicate_property_20180213.

Backed out changes from 9226 which incorrectly changed the types of some opcodes etc. in a couple of functions.

Added minor code to compiler to support the form:
p(X,---) :- X = f(a,b), ...
to use the f(a,b) to index on the first argument. (Works for any argument.)
Finally can get indexing *and* avoid reconstructing the term on the heap.
This is not applied to tabled predicates (since it causes a segfault.)
I should try to figure out why. I suspect there might be a program
transformation that it's intefering with.

allow hooks to have non-atomic types

Much new documentation.
Small small optimizations.
A few other changes.

Added write_canonical_lettervar/2, to be consistent with all other
write-like predicates, to handle open files *and* userout in first arg.