WASM Support

[jesseditson]

• add a time module at the root with a now implementation that is replaced with a js_sys call when compiling for wasm32
• use Instant from web_time when targeting wasm32
• add js_sys and web_time dependencies when targeting wasm32

Motivation

I was attempting to use this library with a wasm target in a browser. If you call std::time::{Instant, SystemTime}::now in a wasm target, it will panic at runtime. This library makes use of both ::now() calls, which will result in a panic when attempting to record a trace.

Solution

In the library that this library calls (opentelemetry-api), this is resolved by replacing calls to SystemTime::now() with an implementation that returns a SystemTime constructed from js_sys::Date. To preserve the ability to pass SystemTime-typed args to opentelemetry_api methods, I just added the same mechanism to this library.

Because Instant is never used in opentelemetry_api, we can instead replace the implementation entirely with the api-compatible "polyfill" from web_time.

All of these changes are behind cfg and automatically enabled when building for wasm32 targets, so this will not change any behavior/performance when built for other targets.

[jtescher]

Looks good, thanks @jesseditson