libexpr/primops: Migrate addPath to return a Value

libexpr/primops: Migrate derivationStrictInternal to return a Value

libexpr/primops: Migrate helper fn fetch to return a Value

libexpr/primops: Migrate helper fn fetchTree to return a Value

libexpr/primops: Migrate helper fn anyorall to return a Value

libexpr: Migrate EvalState::concatLists to return a Value

libexpr/primops: Migrate helper fn elemAt to return a Value

libexpr/json-to-value: Migrate parseJSON to return a Value

lixexpr/primops: Migrate import helper fn to return a Value

libexpr: Migrate EvalPaths::allowAndSetStorePathString to return a Value

repl: Fix the `use` command

improve justfile, and add more docs about it

f2/nix: rename `_serialise` to `_serialise_config`

f2/testlib: fix typechecking for None values

docs: add section about overlays in `nix repl --help`