Custom INI-esque parser for repart config files

[korewaChino]

This PR adds a custom systemd-repart config parser which allows for duplicate keys inside the config files, i.e

[Partition]
Type=root
Format=btrfs
Subvolumes=/@ /@home
MountPoint=/:rw,seclabel,relatime,compress=zstd:1,ssd,discard=async,space_cache=v2,subvolid=257,subvol=/@
MountPoint=/home:rw,seclabel,relatime,compress=zstd:1,ssd,discard=async,space_cache=v2,subvolid=256,subvol=/@home
FactoryReset=yes
# This path is actually relative; see man repart.d at --copy-source option
CopyFiles=/:/
ExcludeFiles=/boot/

It does this by storing the values as an enum of either a normal string or a Vec of strings, converting when multiple duplicate keys are found.

I don't know if it would be useful in place of the original serde_ini implementation,. but it kind of fixes the issue behind BTRFS subvolume mounts in #34

[madonuko]

can we test this build of readymade?