Pkg checksums rework

A quick summary of the digests changes:

- checksums now contain version and digests_suite id which includes hashing and encoding functions
- all checksums operations are now in `pkg_checksum.c`
- checksums type can now be found in repo meta
- if a checksum is in a wrong format, `pkg` tries to upgrade it
- a compatibility layer for pkg 1.2 repos has been added