Genericize Content to allow AsRef<[Operation]>

[z33ky]

This was just Vec<Operation> before, which may be the most common use-case and is also required for parsing, since allocation is necessary.
When creating a document allocating a Vec per Content may not be necessary though, so for that use-case allowing any AsRef<[Operation]>, like just a slice, is convenient.

Example:

let content = Content {
	operations: /* note the missing vec!*/[
		Operation::new("BT", vec![]),
		Operation::new("Tf", vec!["F1".into(), 48.into()]),
		Operation::new("Td", vec![100.into(), 600.into()]),
		Operation::new("Tj", vec![Object::string_literal("Hello World!")]),
		Operation::new("ET", vec![]),
	],
};

The operands for Operation still require Vec. If the Operation struct were to be replaced with an enum as suggested in #109 (with an additional Custom { operation: AsRef<str>, operands: AsRef<[Object]> }) that would become unnecessary though. If you like the idea I'd be happy to make a PR for that as well.

[z33ky]

I'm not sure if having this default is desirable, but that should avoid downstream breakages.
If not now, then perhaps in a future version when a breaking change is added anyways, this can be removed if deemed inappropriate.