`tendermint-testgen`: add support for `no_std`

[plafer]

Description

It would be great if the tendermint-testgen crate supported no_std. This is currently a blocker in ibc-rs for making our mocks feature no_std.

Definition of "done"

tendermint-testgen = {version = "0.x", default-features = false }

doesn't link std.

[thanethomson]

@plafer do you perhaps have capacity to implement this?

[plafer]

Yes, happy to do it whenever I find the time. However, note that this is not high priority in ibc-rs right now.