Add generic functions to must package, including new functions ContainSubset and NotContainSubset. Generic functions now mean EqualStr is renamed Equal.
must functions now typically accept gjson.Result and not byte slices.
JSON matchers now work with gjson.Result and not byte slices.
Tidy up tests to use more matchers than boilerplate.
mustandmatchpublic.mustpackage, including new functionsContainSubsetandNotContainSubset. Generic functions now meanEqualStris renamedEqual.mustfunctions now typically acceptgjson.Resultand not byte slices.gjson.Resultand not byte slices.