- Can depend conditionally based on variant, compiler, arch, deps, etc
- normalize() is not iterative yet: no chaining depends_ons
- really need a SAT solver, but iterative will at least handle
simple cases.
- Added "strict" option to Spec.satisfies()
- strict checks that ALL of other's constraints are met (not just
the ones self shares)
- Consider splitting these out into two methods: could_satisfy() and
satisfies()
- didn't do this yet as it would require changing code that uses
satisfies()
- Changed semantics of __contains__ to use strict satisfaction (SPACK-56)
- Added tests for optional dependencies.
- The constrain() method on Specs, compilers, versions, etc. now
returns whether the spec changed as a result of the call.
