util/order.jam (and engine/mod_order.cpp) review

[paolopas]

• restored Jam add-pair method, for better error control (Jam does a syntax check for arguments at call site with relative error message emission) and better preconditions checkings too: avoid null arguments, avoid duplicate constraints (which led to edge duplication in final graph,) and avoid self referencing (which led to cycles in final graph.) The old add-pair was even broken, since it spelled .constraits instead of .constraints
• removed stale order method implementation, fully replaced by native one
• removed all other stale methods, no more useful/functional

mod_order.cpp rewritten

• to implement the topological sort algorithm using std::vector instead of mem.h
• also removed jam_strings.h include
• prepared for future warning emission on "cyclic dependency"^[1]
• no more need for native add_pair

[1]

I am waiting for #581 to be able to use the outerr warning function.

[paolopas]

Well, the 237df2b tests went fine, so there were no problems with Boost ≥ 1.66.

Important

René, if you think there might be backwards compatibility issues due to the removal of the native add-pair rule (which was required in order.jam without a HAS_NATIVE_RULE guard), then please add 925cbc5 which restores it otherwise don't include it.

Removing the native add-pair could create problems if a new engine (without the native add-pair) deals with old jamfiles, which require it (without checking for its existence.)