Provide more robust impls of Frame::ToFrameBody

[zslayton]

This trait is defined for String, &str and &[u8]. It feels like there should be a way to write an elegant generic impl to cover a variety of classes using AsRef and similar, but initial efforts were fruitless. This was in part due to rustc's refusal to allow more than one generic implementation of a trait for fear that the resulting code will be ambiguous.

[zslayton]

This will likely become much easier when impl specialization lands.