Add support for pointers to builtin primitives

[jgallagher]

This came up in omicron, where we wanted to add a probe for raw bytes. A workaround is to use integers; e.g.,

#[usdt::provider(provider = "blahblah")]
mod probes {
    fn raw_bytes(
        _data: u64, // TODO actually a `*const u8`, but that isn't currently allowed by usdt
        _len: u64, // length of data
    ) {}
}

which functions correctly from the dtrace side but is a little awkward on the Rust side.

[bnaecker]

Closed by #56