Fix the sig parsing in narinfo parsing code

Unlike references, multiple signatures are stored as separate key-value
pairs. Previously this crate assumed that narinfo sigs were stored as a
single key, separated by spaces.

src/narinfo/from_str.rs

@@ -23,6 +23,8 @@ impl<'a> NarInfo<'a> {
 	/// ```
 	pub fn parse(value: &'a str) -> ParsingResult<Self> {
 		let mut builder = NarInfoBuilder::default();
+		let mut sigs = Vec::new();
+
 		for line in value.lines() {
 			let (key, value) = line
 				.split_once(':')
@@ -46,20 +48,15 @@ impl<'a> NarInfo<'a> {
 				"Deriver" => builder.deriver(Some(Cow::from(value))),
 				"System" => builder.system(Some(Cow::from(value))),
 				"References" => builder.references(value.split(' ').map(Cow::from).collect()),
-				// TODO: replace with try_collect once that gets stabilized
-				"Sig" => builder.sigs(value.split(' ').map(Sig::try_from).try_fold(
-					Vec::new(),
-					|mut a, c| {
-						c.map(|c| {
-							a.push(c);
-							a
-						})
-					},
-				)?),
+				"Sig" => {
+					sigs.push(Sig::try_from(value)?);
+					&mut builder
+				},
 				_ => return Err(ParsingError::UnknownKey { key }),
 			};
 		}
 
+		builder.sigs(sigs);
 		builder.build()
 	}
 }