Replace the `.ji` serialization with sysimage format
This unifies two serializers, `dump.c` (used for packages)
and `staticdata.c` (used for system images). It adopts the
`staticdata` strategy, adding support for external linkage,
uniquing of MethodInstances & types, method extensions,
external specializations, and invalidation. This lays the
groundwork for native code caching as done with system images.
Co-authored-by: Valentin Churavy <v.churavy@gmail.com>
Co-authored-by: Jameson Nash <vtjnash@gmail.com>
Co-authored-by: Tim Holy <tim.holy@gmail.com>
