Pass long strings as [ubyte] on code_fetch

[kitsonk]

Long strings (e.g. source_code, output_code) should be passed as binary [ubyte] instead of as a string. Since JS will decode the flatbuffer into a string anyway, having these be a string means Rust must also decode utf8.

CodeFetchRes contains some of these:

deno/src/msg.fbs

Lines 142 to 151 in a68403d

	table CodeFetchRes {
	// If it's a non-http module, moduleName and filename will be the same.
	// For http modules, moduleName is its resolved http URL, and filename
	// is the location of the locally downloaded source code.
	module_name: string;
	filename: string;
	media_type: MediaType;
	source_code: string;
	output_code: string; // Non-empty only if cached.
	}