Supply instant::SystemTime for wasm32

`std::time::SystemTime` is unavailable in wasm32-unknown-unknown.

Cargo.toml

@@ -15,6 +15,12 @@ default = ["alloc"]
 alloc = []
 std = ["alloc"]
 
+[target.'cfg(all(target_family = "wasm", target_os = "unknown"))'.features]
+web = ["web-time"]
+
+[target.'cfg(all(target_family = "wasm", target_os = "unknown"))'.dependencies]
+web-time = { version = "1", optional = true }
+
 [package.metadata.docs.rs]
 all-features = true
 rustdoc-args = ["--cfg", "docsrs"]

src/lib.rs

@@ -52,8 +52,13 @@ use alloc::vec::Vec;
 use core::fmt;
 use core::ops::Deref;
 use core::time::Duration;
-#[cfg(feature = "std")]
+#[cfg(all(
+    feature = "std",
+    not(all(target_family = "wasm", target_os = "unknown", feature = "web"))
+))]
 use std::time::SystemTime;
+#[cfg(all(target_family = "wasm", target_os = "unknown", feature = "web"))]
+use web_time::SystemTime;
 
 mod server_name;
 pub use server_name::{
@@ -478,7 +483,10 @@ pub struct UnixTime(u64);
 
 impl UnixTime {
     /// The current time, as a `UnixTime`
-    #[cfg(feature = "std")]
+    #[cfg(any(
+        feature = "std",
+        all(target_family = "wasm", target_os = "unknown", feature = "web")
+    ))]
     pub fn now() -> Self {
         Self::since_unix_epoch(
             SystemTime::now()