Core probably should not use context.Context

[jsccast]

context.Context depends (indirectly) on fmt and bunch of other stuff. To keep core small, remove that explicit context dependency by introducing a local Context type that satisfies the right interface.

[jsccast]

Also consider not using campoy/jsonenums because that code also pulls in fmt.

[jsccast]

encoding.json of course depends on fmt, and removing our dependency on encoding.json -- though small -- isn't easy. (Spec.PatternSyntax == "json" is just a bit too convenient to have right there. At least for now.)