The new version ofget_variable returns the required size in the error data if the input buffer is too small. This allowsget_variable_boxed to usemake_boxed, and also makesget_variable_size unnecessary.

Also:

• Added more info about errors to the docstrings.
• implAlign for[u8] to allowmake_boxed to work here.

(Note:variable_keys function to come in a later PR.)

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See theRewriting History guide for help.
• Update the changelog (if necessary)