Evaluate argument to instant/atomic atomically, as a special case

[byorgey]

Since Swarm is strict, the way an application instant c works is as follows:

1. Evaluate the LHS instant to a value (immediately succeeds since it is just a constant)
2. Evaluate the RHS c to a value
3. Perform the function application instant c, by atomically executing the command represented by c

Usually, step (2) succeeds almost instantly as well. For example if c is something like move; move; turn right; move then it just instantly turns into a command value. However, c could be something like def x = (expensive pure expression) end; other stuff in which case evaluating c will cause the definition of x to be (expensively) evaluated.

This PR rectifies the situation by ensuring that not only are instant/atomic executed atomically, but also their argument is evaluated atomically. Fixes #2270.