Release v0.1.21

There is an installation issue reported in #11 (comment)
It seems that these binaries need to be rebuilt and then re-uploaded here.

For example:
a) the name of the compressed binary must be rustwasmc instead of ssvmup (so that the URL for the fetch in the installer is correct)
b) the actual root dir of the repo (once uncompressed) needs to be rustwasmc instead of ssvmup or the installer will break.