resolvo_cpp-v0.2.0

Added

• More tracing (#55)
• (breaking) Version set unions as solvable requirements (#56)
• (breaking) Solve for optional solvables in addition to the root solvable (#54)
• (breaking) Add Problem struct (#62)
• (breaking) Decide on explicit requirements first (#61)

Fixed

• Display_merged_solvables to display merged solvables correctly (#48)
• Add a virtual destructor to DependencyProvider (#50)
• Fix off-by-one error in Mapping::serialize (#58)

Performance

• Visit less clauses during propagation (#66)
• Implement a form of VSIDS (#67)