ir: make HashCode.toHashString public

This will allow chiseltest to save
the hash code to disk for the purpose
of caching simulation binaries.

src/main/scala/firrtl/ir/StructuralHash.scala

@@ -115,16 +115,18 @@ object StructuralHash {
 }
 
 trait HashCode {
-  protected val str: String
-  override def hashCode(): Int = str.hashCode
+  /** String representation of the hash code.
+    * Two instances of [[HashCode]] are equal if and only if their toHashString values are equal. */
+  def toHashString: String
+  override def hashCode(): Int = toHashString.hashCode
   override def equals(obj: Any): Boolean = obj match {
-    case hashCode: HashCode => this.str.equals(hashCode.str)
+    case hashCode: HashCode => this.toHashString.equals(hashCode.toHashString)
     case _ => false
   }
 }
 
 private class MDHashCode(code: Array[Byte]) extends HashCode {
-  protected override val str: String = code.map(b => f"${b.toInt & 0xff}%02x").mkString("")
+  override val toHashString: String = code.map(b => f"${b.toInt & 0xff}%02x").mkString("")
 }
 
 /** Generic hashing interface which allows us to use different backends to trade of speed and collision resistance */

src/test/scala/firrtl/ir/StructuralHashSpec.scala

@@ -38,6 +38,16 @@ class StructuralHashSpec extends AnyFlatSpec {
     assert(hash(b1) != hash(UIntLiteral(1, IntWidth(2))))
   }
 
+  it should "generate the same hash String if the objects are structurally the same" in {
+    assert(hash(b0).toHashString == hash(UIntLiteral(0, IntWidth(1))).toHashString)
+    assert(hash(b0).toHashString != hash(UIntLiteral(1, IntWidth(1))).toHashString)
+    assert(hash(b0).toHashString != hash(UIntLiteral(1, IntWidth(2))).toHashString)
+
+    assert(hash(b1).toHashString == hash(UIntLiteral(1, IntWidth(1))).toHashString)
+    assert(hash(b1).toHashString != hash(UIntLiteral(0, IntWidth(1))).toHashString)
+    assert(hash(b1).toHashString != hash(UIntLiteral(1, IntWidth(2))).toHashString)
+  }
+
   it should "ignore expression types" in {
     assert(hash(add) == hash(DoPrim(Add, Seq(b0, b1), Seq(), UnknownType)))
     assert(hash(add) == hash(DoPrim(Add, Seq(b0, b1), Seq(), UIntType(UnknownWidth))))