Support for using cargo-binstall to install wasmer

[NobodyXu]

cargo-binstall provides a simple way to install prebuilt rust binaries.

Supporting it is not that hard consider that wasmer already put prebuilt binaries onto its release page.

The limitation to cargo-binstall is that it cannot install WASMER_DIR, so either we can propose a new feature to cargo-binstall or wasmer can support downloading WASMER_DIR required for wasmer create-exe.

[NobodyXu]

@epilys If the WASMER_DIR part is doable, then can we also add support for cargo-binstall?

It's also quite easy.

[NobodyXu]

I might submit a PR for this today.