Set default R for Context

[jonhoo]

If we expect users to often end up with Context<EndianRcSlice<RunTimeEndian>>, should we just make that the default type? As in:

pub struct Context<R = EndianRcSlice<RunTimeEndian>> {}

That would save users of the library to explicitly type out the full type for Context in the common case!

[philipc]

Makes sense since that's what Context::new returns. You can also do a type alias in your code though.

[jonhoo]

Oh, sure, I could totally do it in my code, but I guess the question is whether it makes sense to just include it directly in addr2line?

[philipc]

Fixed by #111