wasm32-unknown-emscripten support

[yu-re-ka]

Describe the bug

Even though uuid is included with the v4 feature, the new_v4 function doesn't exist.

error[E0599]: no function or associated item named `new_v4` found for struct `Uuid` in the current scope
  --> src/core/segment_id.rs:43:11
   |
43 |     Uuid::new_v4()
   |           ^^^^^^ function or associated item not found in `Uuid`

To Reproduce
Steps to reproduce the behavior:

1. Add uuid dependency with v4 feature
2. Call new_v4 function
3. Build with target wasm32-unknown-emscripten

Expected behavior

Since emscripten provides the rand functions via the usual C api, uuid should not treat wasm32-unknown-emscripten different than x86_64-unknown-linux-gnu.

It should use libc random via the rand crate as usual.

Screenshots
If applicable, add screenshots to help explain your problem.

Specifications (please complete the following information):

• Target: wasm32-unknown-emscripten
• Version 0.8.2
• Features Enabled: v4

Additional context

#351

Back then nobody thought about emscripten, so they assumed all wasm32 targets would need workarounds.

Other
An exception was already added for wasi:
#477
But really this should be a specific check for wasm32-unknown-unknown and allow everything else.