Disable dependency on instant when std feature is disabled

Cargo.toml

@@ -16,10 +16,10 @@ exclude = ["/.*"]
 
 [features]
 default = ["std"]
-std = []
+std = ["instant"]
 
 [target.'cfg(all(target_arch = "wasm32", not(target_os = "wasi")))'.dependencies]
-instant = "0.1"
+instant = { version = "0.1", optional = true }
 
 [target.'cfg(all(target_arch = "wasm32", not(target_os = "wasi")))'.dev-dependencies]
 instant = { version = "0.1", features = ["wasm-bindgen"] }