Add standalone assertions

[richfitz]

This package bundles up the most recent copy of assertions, as used in odin etc, in a form that we can reuse them elsewhere without needing a dependency (and the complex CRAN interactions that will require when we improve things)

For usage, see:

• Use assertions from reside.utils mrc-ide/odin2#33
• Use standalone assertions from reside.utils mrc-ide/monty#67
• Use standalone assertions mrc-ide/dust2#67