Allow dependencies between pages on the wiki

[robsimmons]

This was a feature I strongly resisted adding in the Mediawiki days, but I think the greatly simplified TwelfTag system we've got going makes it quite a bit more doable.

Also adds type-safety-for-the-stlc as a test of the feature. That file has the header

%%! title: "Type safety for the simply-typed lambda calculus"
%%! import: simply-typed-lambda-calculus.elf

which causes all the Twelf code to be checked as if the contents of simply-typed-lambda-calculus.elf was included before the file. This has #import semantics from Objective-C, or #include C preprocessor semantics where IFDEF is used correctly: multiple dependencies are allowed all transitive dependencies are included, but only once. This would allow there to be one file that contains natural numbers with no imports, a second file that decribes less-than

%%! title: "Inequality for nats"
%%! import: nat.elf

and a third file that describes addition

%%! title: "Addition for nats"
%%! import: nat.elf

then, a fourth file could have the following imports

%%! title: "Addition for nats"
%%! import: nat.elf
%%! import: inequality-nat.elf
%%! import: addition-nat.elf

and

• the nat.elf import is optional, because it will be included as a transitive dependency
• both the inequality-nat.elf and addition.nat definitions will use the same defnition of nat which is only defined once, in nat.elf.

[robsimmons]

Moving the cfg auto-generation into the wiki project means we can just invoke that auto-generation to run the regression tests.