Asdf: Consider Banning echo command

[Stratus3D]

Is your feature request related to a problem? Please describe.

As described in https://unix.stackexchange.com/questions/65803/why-is-printf-better-than-echo/65819#65819, echo has a lot of disadvantages and should never be passed uncontrolled data, which is something we often do.

Describe the solution you'd like

Banning the echo command entirely would be an effective way of avoiding these issues and would force us to be more explicit with printf formatting.

Describe workarounds you've considered

We can continue use echo when it printing controlled data, but the inconsistency may not be caught by reviewers or our code quality tools. Banning the command would ensure these issues are not possible.

[Stratus3D]

Related - http://www.etalabs.net/sh_tricks.html

[jthegedus]

I was going to suggest banning echo on my output refactor/standardisation. printf is definitely my preference here.

[Stratus3D]

@jthegedus https://github.com/asdf-vm/asdf/pull/806 is ready for review now.