support WASM build

[xudesheng]

I want to use this crate in WASM. However, it's blocked by the "std::time" in the dependency of "globalcache". But I couldn't find anywhere to submit a PR for the global cache crate, so I want to put my comment here.

The fix is straightforward:

1. Add "web-time" to the globalcache:

[dependencies]
web-time = "1.1.0"

2. change the sync.rs
   Original code:

use std::time::{Instance, Duration};

New code:

use web_time::{Instant, Duration};

[s3bk]

Thanks. I applied your fix. I think cargo-update should pull in the new version. If not I need to release a new version of pdf.