Track which version introduced a mutation pattern

[sourcefrog]

In short,

• The json version of mutants should say which cargo-mutants release introduced this pattern
• There should be a --patterns-version option to generate only mutants that would have been produced by a given cargo-mutants version.

Doing this with perfect fidelity might be complex, if e.g. we fix bugs causing some mutants to be lost, or when generation depends on the interaction of several features. However, we could have tests asserting that particular trees generate particular mutants with certain pattern versions, and then freeze that output.

Discussed in #228

Related: #224 about documenting stability.