Replace C-style term type integer checks with TermType enum

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This PR:

• adds the TermType enum, which allows exhaustive term type matching;
• adds IStrategoTerm.getType(), which returns a member of the TermType enum;
• deprecates IStrategoTerm.getTermType(), as it returns an integer value (remnant of C-style 'enums');
• deprecates the IStrategoTerm APPL, LIST, INT, REAL, STRING, CTOR, TUPLE, REF, BLOB and PLACEHOLDER integer constants;
• replaces many uses of getTermType() with getType().

This PR allows you to write switch on IStrategoTerm.getType() without having to prefix every case with IStrategoTerm, or having to do a static import for each member. In languages that support it (e.g., Kotlin), the compiler can detect that the switch-case is exhaustive. As enums are safely compared using reference equality, this PR will have no performance impact (comparing two pointers is the same as comparing two integers).

Related:

• metaborg/mb-exec#9
• metaborg/nabl#33
• metaborg/flowspec#5
• metaborg/jsglr#82
• metaborg/strategoxt#33

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What's the Jenkinsfile for?

I think Gabriel added that to trigger a build, since the old triggers stopped working.