Add smartstring default feature.

Cargo.toml

@@ -17,13 +17,12 @@ categories = ["no-std", "embedded", "wasm", "parser-implementations"]
 
 [dependencies]
 smallvec = { version = "1.6", default-features = false, features = ["union"] }
-smartstring = { version = "0.2.6" }
 ahash = { version = "0.7", default-features = false }
 num-traits = { version = "0.2", default_features = false }
 rhai_codegen = { version = "0.3.4", path = "codegen", features = ["metadata"] }
 
 [features]
-default = []
+default = ["smartstring"]
 unchecked = []                  # unchecked arithmetic
 sync = []                       # restrict to only types that implement Send + Sync
 no_optimize = []                # no script optimizer
@@ -76,6 +75,10 @@ default_features = false
 features = ["alloc"]
 optional = true
 
+[dependencies.smartstring]
+version = "0.2.6"
+optional = true
+
 [dependencies.unicode-xid]
 version = "0.2"
 default_features = false