Avoid initialization when validating many instances against same schema

[nichtich]

I'd expect a way to initialize a schema only once and validate many instances against it, e.g:

my $validator = JSV::Validator->compile($schema);
say $validator->validate($instance) for @instances;

As I read the code, a lot of initialization is done for each instance in JSV::Validator::validate and JSV::Context::validate.