First working Makefile generation

t/samples/01 now runs if you do

    ./runhopen --fresh && ./runhopen && (cd built && make)

General:
- Hard-code the BH::Conventions URL in README.md since I don't
  have a non-TRIAL release on MetaCPAN yet.
- Slightly expand BH::Conventions
- Bump version

Scope:
- Use the name "context" to represent the outer scope
- Split run() into Scope::run() and <subclass>::_run()

DAG:
- Clean up DAG::_run()
- Run Links in series, not parallel
- Don't create Link if we don't need one

G:
- Change PassthroughOp to more flexible CollectOp, e.g., for use in t/021.

T and Gen:
- Regularize the use of `work`
- Use the right paths for the source and EXE files

App:
- Add --verbose option

Tests:
- Small updates based on changes above
- 006: Add test of local on G::PassthroughOp
- 006, 021: add levels

Prereqs:
- Bump File::Glob minimum version, since I was seeing failures on 5.14.x
  due to lack of `:bsd_glob`.  perl5160delta says 5.16 used File::Glob 1.17,
  so that's the new minimum requirement.