PATH-like variable modifications overwrite instead of union in saves

[whisperity]

Reproduce:

1. ep +PATH foo
2. ep save foo
3. ep +PATH bar
4. ep save foo
5. cat ~/.local/share/envprobe/saves/foo.json

Results in the following save:

{
  "unset": [],
  "variables": {
    "PATH": {
      "add": [
        "/tmp/bar"
      ],
      "remove": []
    }
  }

The addition of directory foo to the variable is missing.

[whisperity]

envvar has been extended with apply_diff and merge_diff on the interface, lowering the responsibility into the typeclass instead of the save and load commands, which now use these methods to properly "concatenate" changes.