Support jsonnet >= v0.19.0

[eagletmt]

jsonnet v0.19.0 changed the C API for import callbacks. https://github.com/google/jsonnet/releases/tag/v0.19.0

This PR should fix #37

[kaorimatz]

I think we are expected to return arbitrary bytes instead of c string (null terminated) with buf because importbin may import data containing null bytes, so we may need another helper function like below.

char *
rubyjsonnet_str_to_ptr(struct JsonnetVm *vm, VALUE str)
{
    const char *const ptr = StringValuePtr(str);
    char *const buf = jsonnet_realloc(vm, NULL, RSTRING_LEN(str));
    memcpy(buf, ptr, RSTRING_LEN(str));
    return buf;
}

https://github.com/google/jsonnet/blob/v0.19.0/include/libjsonnet.h#L72

before:

irb(main):001:0> vm = Jsonnet::VM.new
=> #<Jsonnet::VM:0x0000753e9e129b50>
irb(main):002:0> vm.import_callback = ->(base, rel) { ["\x0\x1", base + rel] }
=> #<Proc:0x0000753e9f670310 (irb):2 (lambda)>
irb(main):003:0> vm.evaluate('importbin "foo"')
/path/to/ruby-jsonnet/lib/jsonnet/vm.rb:73:in `eval_snippet': string contains null byte (ArgumentError)

after:

irb(main):001:0> vm = Jsonnet::VM.new
=> #<Jsonnet::VM:0x00007690148dc280>
irb(main):002:0> vm.import_callback = ->(base, rel) { ["\x0\x1", base + rel] }
=> #<Proc:0x0000769016a248e8 (irb):2 (lambda)>
irb(main):003:0> vm.evaluate('importbin "foo"')
=> "[\n   0,\n   1\n]\n"