add optional smol_str support (#72)

Co-authored-by: Graham Esau <gesau@hotmail.co.uk>

schemars/tests/expected/smol_str.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "String",
+  "type": "string"
+}

schemars/tests/smol_str.rs

@@ -0,0 +1,8 @@
+mod util;
+use smol_str::SmolStr;
+use util::*;
+
+#[test]
+fn smol_str() -> TestResult {
+    test_default_generated_schema::<SmolStr>("smol_str")
+}