Remove Context::new

[philipc]

The use of object in the public API means that breaking changes in object are breaking changes in this crate, and object necessarily has a large evolving API so breaking changes in it will continue to occur. It would be nice to avoid this.

A minor additional issue is that Context::new uses EndianRcSlice which isn't good for performance and memory usage.

Context::new is fairly small, and some of the major users of this crate already don't use it. If required, we could move it to a new crate. Such a crate may then even provide a higher level API, e.g. given a path to an executable, it could find the debug info using moria and load the file. This would address #110.

[philipc]

This is complicated because builtin_split_dwarf_loader depends on object now too, so it would have to move to a new crate.