[ecash-lib] Move ecash-lib-wasm back to root workspace

Summary: This was previously removed as it broke the build, now we re-enable it again.

Test Plan: @bot build-linux32

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16052