app: store values with string representation

Improves code readability without changing memory layout.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/strings.go

@@ -0,0 +1,19 @@
+package app
+
+import (
+	"strconv"
+
+	"git.gensokyo.uk/security/fortify/fst"
+)
+
+func newInt(v int) *stringPair[int]        { return &stringPair[int]{v, strconv.Itoa(v)} }
+func newID(id *fst.ID) *stringPair[fst.ID] { return &stringPair[fst.ID]{*id, id.String()} }
+
+// stringPair stores a value and its string representation.
+type stringPair[T comparable] struct {
+	v T
+	s string
+}
+
+func (s *stringPair[T]) unwrap() T      { return s.v }
+func (s *stringPair[T]) String() string { return s.s }