Part 1 of splitting #13.

This refactors the layout of the crate and cleans up imports a bit, but no functional changes.

• Puts the linux/android specific code into a linux submodule that is reexported from the root so the outward API doesn't really change, it will just make adding additional target platforms cleaner
• Renamed LinuxPtraceDumper to PtraceDumper, the naming was inherited from breakpad but doesn't make much sense since there is only one ptrace implementation, and even if a future one was added eg BSD, it would either be its own separate one, or else a specialization of the linux one
• There are quite a few target specific parts of this code requiring relatively complicated cfg expressions, so I added cfg-if to make them cleaner and easier to read
• A lot of the imports where massive with lots of duplication, so I collapsed most of them
• Cleaned up the names of nested modules as they were needlessly prefixed with the parent module name
• Removed all of the empty mips/ppc/sparc implementations

One breaking change I made was to move to the 2021 edition which is 5 months old at this point, but I can revert that if it's not wanted, the only change that this really enabled was to remove all of the std::convert::TryInto imports since that is part of the prelude in the 2021 edition.