feat(nix-compat/narinfo/signature): generalize name field

Requiring `name` to be a `&str` means it'll get annoying to pass around
`Signature`, but being able to pass them around in an owned fashion is
kinda a requirement for a stronger typed `PathInfo` struct, where we
want to have full ownership.

Rework the `Signature` struct to become generic over the type of the
`name` field. This means, it becomes possible to have owned versions
of it.

We don't want to impose `String` or `SmolStr` for example, but want to
leave it up to the nix-compat user to decide.

Provide a type alias for the existing `&str` variant (`SignatureRef`),
and use it where we previously used the non-generic `Signature` one.

Add some tests to ensure it's possible to *use* `Signature` with both
`String` and `SmolStr` (but only pull in `smol_str` as dev dependency
for the tests).

Also, add some more docstrings, these were a bit sparse.

Change-Id: I3f75691498c6bda9cd072d2d9dac83c4f6c57287
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12253
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/Cargo.lock

@@ -2357,6 +2357,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sha2",
+ "smol_str",
  "thiserror",
  "tokio",
  "tokio-test",
@@ -2917,7 +2918,7 @@ checksum = "5bb182580f71dd070f88d01ce3de9f4da5021db7115d2e1c3605a754153b77c1"
 dependencies = [
  "bytes",
  "heck",
- "itertools 0.10.5",
+ "itertools 0.13.0",
  "log",
  "multimap",
  "once_cell",
@@ -2950,7 +2951,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "18bec9b0adc4eba778b33684b7ba3e7137789434769ee3ce3930463ef904cfca"
 dependencies = [
  "anyhow",
- "itertools 0.10.5",
+ "itertools 0.13.0",
  "proc-macro2",
  "quote",
  "syn 2.0.48",
@@ -3878,9 +3879,9 @@ checksum = "e6ecd384b10a64542d77071bd64bd7b231f4ed5940fba55e98c3de13824cf3d7"
 
 [[package]]
 name = "smol_str"
-version = "0.2.1"
+version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e6845563ada680337a52d43bb0b29f396f2d911616f6573012645b9e3d048a49"
+checksum = "dd538fb6910ac1099850255cf94a94df6551fbdd602454387d0adb2d1ca6dead"
 dependencies = [
  "serde",
 ]