Support WASM target (or clarify in FAQs)

[lordi]

First of all, thanks for publishing this genius library.

Due to various dependencies (described here), this library can currently not used to build WASM targets. This issue is intended to track the WASM support for typetag specifically (because dependencies might change).

If this issue is a WONTFIX, please add a clarification note to the README FAQs stating that WASM is not supported. This might help developers that would build projects with typetag only to realize later that WASM target is not supported.