- move native backend to pkg/proc/native
- move gdbserver backend to pkg/proc/gdbserial
- move core dumps backend to pkg/proc/core

Core files contain a variety of memory mappings either to files or
anonymous regions stored in the core file. These regions can overlap, so
figuring out what exactly to read can be tricky. This commit contains
a data structure, SplicedMemory, which accumulates mappings and reads
from the correct sources.